feat(01-02): create base model and enums

- Add Base class using SQLAlchemy 2.0 DeclarativeBase
- Add DiscussionType enum (OPEN, DISCUSS)
- Add DiscussionStatus enum (ACTIVE, COMPLETED)
- Add RoundType enum (PARALLEL, SEQUENTIAL)
- Use str-based enums for database portability

src/moai/core/models.py

@@ -0,0 +1,40 @@
+"""SQLAlchemy models for MoAI multi-AI discussion platform.
+
+Data model hierarchy:
+    Project (has many) → Discussion (has many) → Round (has many) → Message
+                      ↘ Discussion (has one) → Consensus
+
+All IDs use UUID stored as String(36) for SQLite compatibility.
+Enums are stored as strings for database portability.
+"""
+
+from enum import Enum
+
+from sqlalchemy.orm import DeclarativeBase
+
+
+class Base(DeclarativeBase):
+    """Base class for all SQLAlchemy models."""
+
+    pass
+
+
+class DiscussionType(str, Enum):
+    """Type of discussion mode."""
+
+    OPEN = "open"
+    DISCUSS = "discuss"
+
+
+class DiscussionStatus(str, Enum):
+    """Status of a discussion."""
+
+    ACTIVE = "active"
+    COMPLETED = "completed"
+
+
+class RoundType(str, Enum):
+    """Type of round in a discussion."""
+
+    PARALLEL = "parallel"
+    SEQUENTIAL = "sequential"