refactor: Update playlist service and endpoints for global current playlist management

app/repositories/playlist.py

@@ -66,6 +66,18 @@ class PlaylistRepository(BaseRepository[Playlist]):
             logger.exception("Failed to get current playlist for user: %s", user_id)
             raise
 
+    async def get_global_current_playlist(self) -> Playlist | None:
+        """Get the global current playlist (app-wide)."""
+        try:
+            statement = select(Playlist).where(
+                Playlist.is_current == True,  # noqa: E712
+            )
+            result = await self.session.exec(statement)
+            return result.first()
+        except Exception:
+            logger.exception("Failed to get global current playlist")
+            raise
+
     async def search_by_name(
         self, query: str, user_id: int | None = None,
     ) -> list[Playlist]: