MetroList: Validate Song Existence Before Adding to Playlist

Problem

Adding songs to playlists could crash if the song didn't exist in the database. Race conditions during sync could remove songs while add operations were in flight.

Solution

Added validation check in the playlist add flow to verify song existence before attempting the database insert. Returns appropriate error if song is missing.

Result

No more crashes from playlist add operations on missing songs. Graceful handling of concurrent sync operations.

Commit: MetrolistGroup/Metrolist@21b3742
PR: #3432