Refactor Playlist model and add timestamps migration

Simplifies the PlaylistBase id field to a non-optional int and updates database logic to use session.refresh for id assignment. Adds Alembic migration to include created_at and updated_at columns in room_playlists for timestamp tracking.

app/database/playlists.py

@@ -26,11 +26,7 @@ if TYPE_CHECKING:
 
 
 class PlaylistBase(SQLModel, UTCBaseModel):
-    id: int | None = Field(
-        default=None,
-        primary_key=True,
-        index=True,
-    )
+    id: int = Field(index=True)
     owner_id: int = Field(sa_column=Column(BigInteger, ForeignKey("lazer_users.id")))
     ruleset_id: int = Field(ge=0, le=3)
     expired: bool = Field(default=False)
@@ -120,11 +116,9 @@ class Playlist(PlaylistBase, table=True):
     async def add_to_db(cls, playlist: PlaylistItem, room_id: int, session: AsyncSession):
         db_playlist = await cls.from_hub(playlist, room_id, session)
         session.add(db_playlist)
-        await session.flush()
-        assert db_playlist.id is not None, "db_playlist.id should be set after flush"
-        playlist.id = db_playlist.id 
         await session.commit()
-
+        await session.refresh(db_playlist)
+        playlist.id = db_playlist.id
 
     @classmethod
     async def delete_item(cls, item_id: int, room_id: int, session: AsyncSession):
@@ -145,4 +139,4 @@ class PlaylistResp(PlaylistBase):
         if "beatmap" in include:
             data["beatmap"] = await BeatmapResp.from_db(playlist.beatmap)
         resp = cls.model_validate(data)
-        return resp
+        return resp