Fix Python 3.8 compatibility: use Tuple instead of tuple in type hints

- Change tuple[...] to Tuple[...] in crud.py line 108
- Add Tuple import from typing
- Fixes TypeError: 'type' object is not subscriptable in Python 3.8

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

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

backend/src/models/crud.py

@@ -1,5 +1,5 @@
 """CRUD operations for audio tracks."""
-from typing import List, Optional, Dict
+from typing import List, Optional, Dict, Tuple
 from uuid import UUID
 from sqlalchemy.orm import Session
 from sqlalchemy import or_, and_, func
@@ -105,7 +105,7 @@ def get_tracks(
     has_vocals: Optional[bool] = None,
     sort_by: str = "analyzed_at",
     sort_desc: bool = True,
-) -> tuple[List[AudioTrack], int]:
+) -> Tuple[List[AudioTrack], int]:
     """Get tracks with filters and pagination.
 
     Args: