feat: Implement favorites management API; add endpoints for adding, removing, and retrieving favorites for sounds and playlists

feat: Create Favorite model and repository for managing user favorites in the database
feat: Add FavoriteService to handle business logic for favorites management
feat: Enhance Playlist and Sound response schemas to include favorite indicators and counts
refactor: Update API routes to include favorites functionality in playlists and sounds

app/models/playlist.py

@@ -5,6 +5,7 @@ from sqlmodel import Field, Relationship
 from app.models.base import BaseModel
 
 if TYPE_CHECKING:
+    from app.models.favorite import Favorite
     from app.models.playlist_sound import PlaylistSound
     from app.models.user import User
 
@@ -23,3 +24,4 @@ class Playlist(BaseModel, table=True):
     # relationships
     user: "User" = Relationship(back_populates="playlists")
     playlist_sounds: list["PlaylistSound"] = Relationship(back_populates="playlist")
+    favorites: list["Favorite"] = Relationship(back_populates="playlist")