feat: run Alembic migrations automatically on startup

- Added alembic==1.18.4 to requirements.txt (was missing, transitive only)
- Added run_alembic_migrations() to startup_event in app.py — runs
   before init_db() to apply all pending migrations
- Combined approach: Alembic for complex migrations first, then inline
  _ensure_*() additive safety net in init_db()
- This ensures DB schema is always up to date on container startup
  without manual intervention

backend/requirements.txt

@@ -37,6 +37,7 @@ referencing==0.37.0
 requests==2.32.5
 rpds-py==0.30.0
 SecretStorage==3.5.0
+alembic==1.18.4
 SQLAlchemy==2.0.45
 starlette==0.50.0
 typing-inspection==0.4.2