Add ID field to Artist and Song structs in the agents package. When resolving
similar artists and top songs, the provider now uses a three-phase lookup:
1. Direct ID match (if agent returns internal Navidrome IDs)
2. MBID exact match (if MusicBrainz ID is available)
3. Fuzzy name/title match (existing behavior)
This enables agents to return more precise matches when they have access to
internal database IDs, while maintaining backward compatibility with
name-based matching.
Deluan