feat(playlists): implement Playlist and PlaylistSound models; add seeding for default Main playlist

app/models/sound.py

@@ -2,14 +2,17 @@
 
 from datetime import datetime
 from enum import Enum
-from typing import Optional
+from typing import TYPE_CHECKING, Optional
 from zoneinfo import ZoneInfo
 
 from sqlalchemy import Boolean, DateTime, Integer, String
-from sqlalchemy.orm import Mapped, mapped_column
+from sqlalchemy.orm import Mapped, mapped_column, relationship
 
 from app.database import db
 
+if TYPE_CHECKING:
+    from app.models.playlist_sound import PlaylistSound
+
 
 class SoundType(Enum):
     """Sound type enumeration."""
@@ -87,6 +90,13 @@ class Sound(db.Model):
         nullable=False,
     )
 
+    # Relationships
+    playlist_sounds: Mapped[list["PlaylistSound"]] = relationship(
+        "PlaylistSound",
+        back_populates="sound",
+        cascade="all, delete-orphan",
+    )
+
     def __repr__(self) -> str:
         """String representation of Sound."""
         return f"<Sound {self.name} ({self.type}) - {self.play_count} plays>"