feat(01-03): apply security middleware stack and database health check

- Add TrustedHostMiddleware for Host header validation
- Add CORSMiddleware with configurable origins
- Add rate limiting with RateLimitExceeded handler
- Add custom middleware for security headers (HSTS, X-Frame-Options, etc.)
- Add /health/db endpoint that checks database connectivity
- Mark health endpoints as rate limit exempt
- Fix linting issues in migration file (Rule 3 - Blocking)

backend/alembic/versions/de1460a760b0_create_build_table.py

@@ -1,48 +1,68 @@
 """Create build table
 
 Revision ID: de1460a760b0
-Revises: 
+Revises:
 Create Date: 2026-01-25 20:11:11.446731
 
 """
-from typing import Sequence, Union
 
-from alembic import op
+from collections.abc import Sequence
+
 import sqlalchemy as sa
-
+from alembic import op
 
 # 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
+revision: str = "de1460a760b0"
+down_revision: str | Sequence[str] | None = None
+branch_labels: str | Sequence[str] | None = None
+depends_on: 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_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)
+    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')
+    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/api/v1/endpoints/health.py

@@ -1,17 +1,44 @@
 """Health check endpoints."""
 
-from fastapi import APIRouter
+from typing import Any
+
+from fastapi import APIRouter, Depends
+from sqlalchemy import text
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from backend.app.api.deps import get_db
+from backend.app.core.security import limiter
 
 router = APIRouter()
 
 
 @router.get("")
+@limiter.exempt
 async def health_check() -> dict[str, str]:
     """Basic health check endpoint."""
     return {"status": "healthy"}
 
 
 @router.get("/ready")
+@limiter.exempt
 async def readiness_check() -> dict[str, str]:
-    """Readiness check endpoint (DB check added in plan 01-02)."""
+    """Readiness check endpoint."""
     return {"status": "ready"}
+
+
+@router.get("/db")
+@limiter.exempt
+async def database_health_check(
+    db: AsyncSession = Depends(get_db),
+) -> dict[str, Any]:
+    """Health check that verifies database connectivity.
+
+    Returns:
+        Status indicating healthy/unhealthy and database connection state.
+    """
+    try:
+        result = await db.execute(text("SELECT 1"))
+        result.scalar()
+        return {"status": "healthy", "database": "connected"}
+    except Exception as e:
+        return {"status": "unhealthy", "database": "error", "detail": str(e)}

backend/app/main.py

@@ -1,9 +1,14 @@
 """FastAPI application entry point."""
 
-from fastapi import FastAPI
+from fastapi import FastAPI, Request, Response
+from fastapi.middleware.cors import CORSMiddleware
+from fastapi.middleware.trustedhost import TrustedHostMiddleware
+from slowapi import _rate_limit_exceeded_handler
+from slowapi.errors import RateLimitExceeded
 
 from backend.app.api.v1.router import api_router
 from backend.app.core.config import settings
+from backend.app.core.security import limiter
 
 app = FastAPI(
     title="Debate API",
@@ -14,6 +19,45 @@ app = FastAPI(
     debug=settings.debug,
 )
 
+# Middleware order matters - first added = outermost layer
+# See: 01-RESEARCH.md Pattern 3: FastAPI Security Middleware Stack
+
+# 1. Trusted Host (reject requests with invalid Host header)
+app.add_middleware(
+    TrustedHostMiddleware,
+    allowed_hosts=settings.allowed_hosts_list,
+)
+
+# 2. CORS (handle cross-origin requests)
+app.add_middleware(
+    CORSMiddleware,
+    allow_origins=settings.allowed_origins_list,
+    allow_credentials=True,
+    allow_methods=["GET", "POST", "PUT", "DELETE", "OPTIONS"],
+    allow_headers=["*"],
+    max_age=600,  # Cache preflight requests for 10 minutes
+)
+
+# 3. Rate limiting
+app.state.limiter = limiter
+app.add_exception_handler(RateLimitExceeded, _rate_limit_exceeded_handler)
+
+
+# 4. Custom middleware for security headers
+@app.middleware("http")
+async def security_headers_middleware(request: Request, call_next: object) -> Response:
+    """Add security headers to all responses."""
+    response: Response = await call_next(request)  # type: ignore[misc]
+    response.headers["Strict-Transport-Security"] = (
+        "max-age=31536000; includeSubDomains"
+    )
+    response.headers["X-Content-Type-Options"] = "nosniff"
+    response.headers["X-Frame-Options"] = "DENY"
+    response.headers["X-XSS-Protection"] = "1; mode=block"
+    response.headers["Referrer-Policy"] = "strict-origin-when-cross-origin"
+    return response
+
+
 # Include API routers
 app.include_router(api_router, prefix="/api/v1")