WIP checkpoint: Address review feedback: Scaffold FastAPI backend project structure

backend/app/database.py

@@ -28,11 +28,15 @@ class Base(DeclarativeBase):
 
 
 async def get_db() -> AsyncSession:
-    """Dependency that yields an async database session."""
+    """Dependency that yields an async database session.
+
+    Handlers are responsible for committing or rolling back their own
+    transactions.  This avoids the double-commit problem where the
+    session is committed both here and in the handler.
+    """
     async with async_session_factory() as session:
         try:
             yield session
-            await session.commit()
         except Exception:
             await session.rollback()
             raise