When Spotify falls back to iTunes/Deezer (auth failure, rate limit, API
error), the manual match modals were storing numeric fallback IDs in
spotify_*_id columns, breaking Spotify links and metadata fetching.
Fix detects the actual provider by inspecting result IDs (alphanumeric =
Spotify, numeric = fallback) rather than checking auth status, which can
be misleading during rate limit bans. The frontend now passes the real
provider to the storage endpoint so IDs land in the correct column.
213736e168