feat(01-02): configure Alembic and create Build model

- Configure Alembic for async migrations with SQLAlchemy 2.0
- Create Build model with UUID primary key, config_hash, status enum
- Add indexes on status (queue queries) and config_hash (cache lookups)
- Generate and apply initial migration creating builds table

Build model fields: id, config_hash, status, iso_path, error_message,
build_log, started_at, completed_at, created_at, updated_at.

backend/alembic.ini

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+# A generic, single database configuration.
+
+[alembic]
+# path to migration scripts.
+# this is typically a path given in POSIX (e.g. forward slashes)
+# format, relative to the token %(here)s which refers to the location of this
+# ini file
+script_location = alembic
+
+# template used to generate migration file names; The default value is %%(rev)s_%%(slug)s
+# Uncomment the line below if you want the files to be prepended with date and time
+# see https://alembic.sqlalchemy.org/en/latest/tutorial.html#editing-the-ini-file
+# for all available tokens
+# file_template = %%(year)d_%%(month).2d_%%(day).2d_%%(hour).2d%%(minute).2d-%%(rev)s_%%(slug)s
+# Or organize into date-based subdirectories (requires recursive_version_locations = true)
+# file_template = %%(year)d/%%(month).2d/%%(day).2d_%%(hour).2d%%(minute).2d_%%(second).2d_%%(rev)s_%%(slug)s
+
+# sys.path path, will be prepended to sys.path if present.
+# defaults to the current working directory.  for multiple paths, the path separator
+# is defined by "path_separator" below.
+prepend_sys_path = .
+
+
+# timezone to use when rendering the date within the migration file
+# as well as the filename.
+# If specified, requires the tzdata library which can be installed by adding
+# `alembic[tz]` to the pip requirements.
+# string value is passed to ZoneInfo()
+# leave blank for localtime
+# timezone =
+
+# max length of characters to apply to the "slug" field
+# truncate_slug_length = 40
+
+# set to 'true' to run the environment during
+# the 'revision' command, regardless of autogenerate
+# revision_environment = false
+
+# set to 'true' to allow .pyc and .pyo files without
+# a source .py file to be detected as revisions in the
+# versions/ directory
+# sourceless = false
+
+# version location specification; This defaults
+# to <script_location>/versions.  When using multiple version
+# directories, initial revisions must be specified with --version-path.
+# The path separator used here should be the separator specified by "path_separator"
+# below.
+# version_locations = %(here)s/bar:%(here)s/bat:%(here)s/alembic/versions
+
+# path_separator; This indicates what character is used to split lists of file
+# paths, including version_locations and prepend_sys_path within configparser
+# files such as alembic.ini.
+# The default rendered in new alembic.ini files is "os", which uses os.pathsep
+# to provide os-dependent path splitting.
+#
+# Note that in order to support legacy alembic.ini files, this default does NOT
+# take place if path_separator is not present in alembic.ini.  If this
+# option is omitted entirely, fallback logic is as follows:
+#
+# 1. Parsing of the version_locations option falls back to using the legacy
+#    "version_path_separator" key, which if absent then falls back to the legacy
+#    behavior of splitting on spaces and/or commas.
+# 2. Parsing of the prepend_sys_path option falls back to the legacy
+#    behavior of splitting on spaces, commas, or colons.
+#
+# Valid values for path_separator are:
+#
+# path_separator = :
+# path_separator = ;
+# path_separator = space
+# path_separator = newline
+#
+# Use os.pathsep. Default configuration used for new projects.
+path_separator = os
+
+# set to 'true' to search source files recursively
+# in each "version_locations" directory
+# new in Alembic version 1.10
+# recursive_version_locations = false
+
+# the output encoding used when revision files
+# are written from script.py.mako
+# output_encoding = utf-8
+
+# database URL.  This is consumed by the user-maintained env.py script only.
+# other means of configuring database URLs may be customized within the env.py
+# file.
+# sqlalchemy.url is set from app.core.config in env.py
+# sqlalchemy.url = driver://user:pass@localhost/dbname
+
+
+[post_write_hooks]
+# post_write_hooks defines scripts or Python functions that are run
+# on newly generated revision scripts.  See the documentation for further
+# detail and examples
+
+# format using "black" - use the console_scripts runner, against the "black" entrypoint
+# hooks = black
+# black.type = console_scripts
+# black.entrypoint = black
+# black.options = -l 79 REVISION_SCRIPT_FILENAME
+
+# lint with attempts to fix using "ruff" - use the module runner, against the "ruff" module
+# hooks = ruff
+# ruff.type = module
+# ruff.module = ruff
+# ruff.options = check --fix REVISION_SCRIPT_FILENAME
+
+# Alternatively, use the exec runner to execute a binary found on your PATH
+# hooks = ruff
+# ruff.type = exec
+# ruff.executable = ruff
+# ruff.options = check --fix REVISION_SCRIPT_FILENAME
+
+# Logging configuration.  This is also consumed by the user-maintained
+# env.py script only.
+[loggers]
+keys = root,sqlalchemy,alembic
+
+[handlers]
+keys = console
+
+[formatters]
+keys = generic
+
+[logger_root]
+level = WARNING
+handlers = console
+qualname =
+
+[logger_sqlalchemy]
+level = WARNING
+handlers =
+qualname = sqlalchemy.engine
+
+[logger_alembic]
+level = INFO
+handlers =
+qualname = alembic
+
+[handler_console]
+class = StreamHandler
+args = (sys.stderr,)
+level = NOTSET
+formatter = generic
+
+[formatter_generic]
+format = %(levelname)-5.5s [%(name)s] %(message)s
+datefmt = %H:%M:%S

backend/alembic/README

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+Generic single-database configuration.

backend/alembic/env.py

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+"""Alembic migration environment configuration for async SQLAlchemy."""
+
+import asyncio
+import sys
+from logging.config import fileConfig
+from pathlib import Path
+
+from alembic import context
+from sqlalchemy import pool
+from sqlalchemy.ext.asyncio import async_engine_from_config
+
+# Add parent directory to path for imports
+sys.path.insert(0, str(Path(__file__).resolve().parents[1]))
+
+from backend.app.core.config import settings  # noqa: E402
+from backend.app.db.base import Base  # noqa: E402
+
+# Import all models for autogenerate to discover them
+from backend.app.db.models import build  # noqa: E402, F401
+
+# Alembic Config object
+config = context.config
+
+# Set sqlalchemy.url from application settings
+config.set_main_option("sqlalchemy.url", settings.database_url)
+
+# Interpret the config file for Python logging
+if config.config_file_name is not None:
+    fileConfig(config.config_file_name)
+
+# SQLAlchemy metadata for autogenerate support
+target_metadata = Base.metadata
+
+
+def run_migrations_offline() -> None:
+    """Run migrations in 'offline' mode.
+
+    This configures the context with just a URL and not an Engine,
+    skipping Engine creation so no DBAPI is needed.
+    """
+    url = config.get_main_option("sqlalchemy.url")
+    context.configure(
+        url=url,
+        target_metadata=target_metadata,
+        literal_binds=True,
+        dialect_opts={"paramstyle": "named"},
+    )
+
+    with context.begin_transaction():
+        context.run_migrations()
+
+
+def do_run_migrations(connection) -> None:
+    """Run migrations with the given connection."""
+    context.configure(connection=connection, target_metadata=target_metadata)
+    with context.begin_transaction():
+        context.run_migrations()
+
+
+async def run_migrations_online() -> None:
+    """Run migrations in 'online' mode with async engine.
+
+    Creates an async Engine and associates a connection with the context.
+    """
+    connectable = async_engine_from_config(
+        config.get_section(config.config_ini_section, {}),
+        prefix="sqlalchemy.",
+        poolclass=pool.NullPool,
+    )
+
+    async with connectable.connect() as connection:
+        await connection.run_sync(do_run_migrations)
+
+    await connectable.dispose()
+
+
+if context.is_offline_mode():
+    run_migrations_offline()
+else:
+    asyncio.run(run_migrations_online())

backend/alembic/script.py.mako

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+"""${message}
+
+Revision ID: ${up_revision}
+Revises: ${down_revision | comma,n}
+Create Date: ${create_date}
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+${imports if imports else ""}
+
+# revision identifiers, used by Alembic.
+revision: str = ${repr(up_revision)}
+down_revision: Union[str, Sequence[str], None] = ${repr(down_revision)}
+branch_labels: Union[str, Sequence[str], None] = ${repr(branch_labels)}
+depends_on: Union[str, Sequence[str], None] = ${repr(depends_on)}
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    ${upgrades if upgrades else "pass"}
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    ${downgrades if downgrades else "pass"}

backend/alembic/versions/de1460a760b0_create_build_table.py

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+"""Create build table
+
+Revision ID: de1460a760b0
+Revises: 
+Create Date: 2026-01-25 20:11:11.446731
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'de1460a760b0'
+down_revision: Union[str, Sequence[str], None] = None
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.create_table('builds',
+    sa.Column('id', sa.UUID(), nullable=False),
+    sa.Column('config_hash', sa.String(length=64), nullable=False),
+    sa.Column('status', sa.Enum('PENDING', 'BUILDING', 'COMPLETED', 'FAILED', 'CACHED', name='buildstatus'), nullable=False),
+    sa.Column('iso_path', sa.String(length=512), nullable=True),
+    sa.Column('error_message', sa.Text(), nullable=True),
+    sa.Column('build_log', sa.Text(), nullable=True),
+    sa.Column('started_at', sa.DateTime(timezone=True), nullable=True),
+    sa.Column('completed_at', sa.DateTime(timezone=True), nullable=True),
+    sa.Column('created_at', sa.DateTime(timezone=True), server_default=sa.text('now()'), nullable=False),
+    sa.Column('updated_at', sa.DateTime(timezone=True), server_default=sa.text('now()'), nullable=False),
+    sa.PrimaryKeyConstraint('id')
+    )
+    op.create_index(op.f('ix_builds_config_hash'), 'builds', ['config_hash'], unique=True)
+    op.create_index('ix_builds_status', 'builds', ['status'], unique=False)
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_index('ix_builds_status', table_name='builds')
+    op.drop_index(op.f('ix_builds_config_hash'), table_name='builds')
+    op.drop_table('builds')
+    # ### end Alembic commands ###

backend/app/db/models/__init__.py

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+"""Database models package.
+
+Import all models here for Alembic autogenerate to discover them.
+"""
+
+from backend.app.db.models.build import Build
+
+__all__ = ["Build"]

backend/app/db/models/build.py

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+"""Build tracking model for ISO generation."""
+
+import enum
+import uuid
+from datetime import datetime
+
+from sqlalchemy import DateTime, Enum, Index, String, Text, func
+from sqlalchemy.dialects.postgresql import UUID
+from sqlalchemy.orm import Mapped, mapped_column
+
+from backend.app.db.base import Base
+
+
+class BuildStatus(enum.Enum):
+    """Status values for build tracking."""
+
+    PENDING = "pending"
+    BUILDING = "building"
+    COMPLETED = "completed"
+    FAILED = "failed"
+    CACHED = "cached"
+
+
+class Build(Base):
+    """Model for tracking ISO build jobs.
+
+    Attributes:
+        id: Unique identifier for the build (UUID)
+        config_hash: SHA-256 hash of the build configuration (64 chars)
+        status: Current build status
+        iso_path: Path to generated ISO file (if completed)
+        error_message: Error message if build failed
+        build_log: Full build output log
+        started_at: Timestamp when build started
+        completed_at: Timestamp when build completed
+        created_at: Timestamp when build was created
+        updated_at: Timestamp of last update
+
+    The config_hash enables caching - identical configurations can
+    return existing ISOs without rebuilding.
+    """
+
+    __tablename__ = "builds"
+
+    # Primary key
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        primary_key=True,
+        default=uuid.uuid4,
+    )
+
+    # Configuration hash for caching (SHA-256 = 64 hex chars)
+    config_hash: Mapped[str] = mapped_column(
+        String(64),
+        unique=True,
+        index=True,
+        nullable=False,
+    )
+
+    # Build status
+    status: Mapped[BuildStatus] = mapped_column(
+        Enum(BuildStatus),
+        default=BuildStatus.PENDING,
+        nullable=False,
+    )
+
+    # Build results
+    iso_path: Mapped[str | None] = mapped_column(
+        String(512),
+        nullable=True,
+    )
+    error_message: Mapped[str | None] = mapped_column(
+        Text,
+        nullable=True,
+    )
+    build_log: Mapped[str | None] = mapped_column(
+        Text,
+        nullable=True,
+    )
+
+    # Timing
+    started_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True),
+        nullable=True,
+    )
+    completed_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True),
+        nullable=True,
+    )
+
+    # Audit timestamps
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        server_default=func.now(),
+        nullable=False,
+    )
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        server_default=func.now(),
+        onupdate=func.now(),
+        nullable=False,
+    )
+
+    # Indexes for common queries
+    __table_args__ = (
+        # Index on status for queue queries (find pending builds)
+        Index("ix_builds_status", "status"),
+        # Index on config_hash already created via column definition
+    )
+
+    def __repr__(self) -> str:
+        """String representation of Build."""
+        return f"<Build {self.id} status={self.status.value}>"