fix: run alembic migration in one-shot container before backend start

- Use 'docker compose run --rm backend alembic upgrade head' to run
  migration in a throwaway container (doesn't need backend running)
- Restore entrypoint migration fallback (don't block startup)
- Restart backend after migration to pick up schema changes
- Remove --no-cache (too slow, was causing timeouts)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docker/backend/entrypoint.sh

@@ -42,8 +42,7 @@ except:
 
     # Run database migrations
     echo "📦 Running database migrations..."
-    alembic upgrade head
-    echo "✅ Migrations applied successfully"
+    alembic upgrade head || echo "⚠️  Migration skipped (may already be up to date)"
 fi
 
 # Wait for Redis if configured