fix(db): add rollback on exception in get_db dependency

Prevents InFailedSQLTransaction cascade — when a request fails mid-
transaction, the connection is rolled back before being returned to
the pool, so subsequent requests on the same connection don't inherit
a poisoned transaction state.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/core/database.py

@@ -27,6 +27,9 @@ async def get_db() -> AsyncSession:
     async with async_session_maker() as session:
         try:
             yield session
+        except Exception:
+            await session.rollback()
+            raise
         finally:
             await session.close()