feat(01-03): add full v1 ORM schema, async session factory, and DB dependency

- backend/db/models.py: 11 SQLAlchemy 2.0 ORM models (User, Quota, RefreshToken,
  Folder, Document, Topic, DocumentTopic, Share, AuditLog, CloudConnection, Group)
- Document.user_id declared nullable=True per D-03 (Phase 2 adds NOT NULL)
- AuditLog.metadata_ uses mapped_column("metadata", JSONB) to avoid DeclarativeBase
  reserved-attribute conflict
- Group table stub for D-02 (v2 feature, seeded per PROJECT.md)
- Uses Optional[X] instead of X | None for Python < 3.10 compatibility
- backend/db/session.py: async engine (pool_pre_ping=True, expire_on_commit=False)
- backend/deps/db.py: async get_db() FastAPI dependency yielding AsyncSession

backend/deps/db.py

@@ -0,0 +1,26 @@
+"""
+FastAPI dependency that yields an async SQLAlchemy session per request.
+
+Usage in route handlers:
+    from deps.db import get_db
+    from sqlalchemy.ext.asyncio import AsyncSession
+    from fastapi import Depends
+
+    @router.get("/items")
+    async def list_items(session: AsyncSession = Depends(get_db)):
+        ...
+"""
+from typing import AsyncGenerator
+
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from db.session import AsyncSessionLocal
+
+
+async def get_db() -> AsyncGenerator[AsyncSession, None]:
+    """Yield a per-request AsyncSession; close it when the request is done."""
+    async with AsyncSessionLocal() as session:
+        try:
+            yield session
+        finally:
+            await session.close()