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/schemas/favorite.py

@@ -0,0 +1,39 @@
+"""Favorite response schemas."""
+
+from datetime import datetime
+
+from pydantic import BaseModel, Field
+
+
+class FavoriteResponse(BaseModel):
+    """Response schema for a favorite."""
+
+    id: int = Field(description="Favorite ID")
+    user_id: int = Field(description="User ID")
+    sound_id: int | None = Field(
+        description="Sound ID if this is a sound favorite", default=None
+    )
+    playlist_id: int | None = Field(
+        description="Playlist ID if this is a playlist favorite", default=None
+    )
+    created_at: datetime = Field(description="Creation timestamp")
+    updated_at: datetime = Field(description="Last update timestamp")
+
+    class Config:
+        """Pydantic config."""
+
+        from_attributes = True
+
+
+class FavoritesListResponse(BaseModel):
+    """Response schema for a list of favorites."""
+
+    favorites: list[FavoriteResponse] = Field(description="List of favorites")
+
+
+class FavoriteCountsResponse(BaseModel):
+    """Response schema for favorite counts."""
+
+    total: int = Field(description="Total number of favorites")
+    sounds: int = Field(description="Number of favorited sounds")
+    playlists: int = Field(description="Number of favorited playlists")