feat(agents): add ID field to Artist and Song structs with direct matching

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.

core/agents/interfaces.go

@@ -22,6 +22,7 @@ type AlbumInfo struct {
 }
 
 type Artist struct {
+	ID   string
 	Name string
 	MBID string
 }
@@ -32,6 +33,7 @@ type ExternalImage struct {
 }
 
 type Song struct {
+	ID   string
 	Name string
 	MBID string
 }