Add multiple features: auth, uploads, queue management, and filters

- Replace email with username for authentication
  - Update User model, schemas, and auth endpoints
  - Update frontend login and register views
  - Add migration to remove email column

- Add multiple track upload support
  - New backend endpoint for bulk upload
  - Frontend multi-file selection with progress
  - Auto-extract metadata from ID3 tags
  - Visual upload progress for each file

- Prevent duplicate tracks in room queue
  - Backend validation for duplicates
  - Visual indication of tracks already in queue
  - Error handling with user feedback

- Add bulk track selection for rooms
  - Multi-select mode with checkboxes
  - Bulk add endpoint with duplicate filtering
  - Selection counter and controls

- Add track filters in room modal
  - Search by title and artist
  - Filter by "My tracks"
  - Filter by "Not in queue"
  - Live filtering with result counter

- Improve Makefile
  - Add build-backend and build-frontend commands
  - Add rebuild-backend and rebuild-frontend commands
  - Add rebuild-clean variants
  - Update migrations to run in Docker

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

backend/app/models/user.py

@@ -11,7 +11,6 @@ class User(Base):
 
     id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
     username: Mapped[str] = mapped_column(String(50), unique=True, nullable=False)
-    email: Mapped[str] = mapped_column(String(255), unique=True, nullable=False)
     password_hash: Mapped[str] = mapped_column(String(255), nullable=False)
     created_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow)