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/external/provider_topsongs_test.go

@@ -271,4 +271,60 @@ var _ = Describe("Provider - TopSongs", func() {
 		ag.AssertExpectations(GinkgoT())
 		mediaFileRepo.AssertExpectations(GinkgoT())
 	})
+
+	It("matches songs by ID first when agent provides IDs", func() {
+		// Mock finding the artist
+		artist1 := model.Artist{ID: "artist-1", Name: "Artist One", MbzArtistID: "mbid-artist-1"}
+		artistRepo.On("GetAll", mock.AnythingOfType("model.QueryOptions")).Return(model.Artists{artist1}, nil).Once()
+
+		// Mock agent response with IDs provided (highest priority matching)
+		// Note: Songs have no MBID to ensure only ID matching is used
+		agentSongs := []agents.Song{
+			{ID: "song-1", Name: "Song One"},
+			{ID: "song-2", Name: "Song Two"},
+		}
+		ag.On("GetArtistTopSongs", ctx, "artist-1", "Artist One", "mbid-artist-1", 2).Return(agentSongs, nil).Once()
+
+		// Mock ID lookup (first query - should match both songs directly)
+		song1 := model.MediaFile{ID: "song-1", Title: "Song One", ArtistID: "artist-1"}
+		song2 := model.MediaFile{ID: "song-2", Title: "Song Two", ArtistID: "artist-1"}
+		mediaFileRepo.On("GetAll", mock.AnythingOfType("model.QueryOptions")).Return(model.MediaFiles{song1, song2}, nil).Once()
+
+		songs, err := p.TopSongs(ctx, "Artist One", 2)
+
+		Expect(err).ToNot(HaveOccurred())
+		Expect(songs).To(HaveLen(2))
+		Expect(songs[0].ID).To(Equal("song-1"))
+		Expect(songs[1].ID).To(Equal("song-2"))
+		artistRepo.AssertExpectations(GinkgoT())
+		ag.AssertExpectations(GinkgoT())
+		mediaFileRepo.AssertExpectations(GinkgoT())
+	})
+
+	It("falls back to MBID when ID is not found", func() {
+		// Mock finding the artist
+		artist1 := model.Artist{ID: "artist-1", Name: "Artist One", MbzArtistID: "mbid-artist-1"}
+		artistRepo.On("GetAll", mock.AnythingOfType("model.QueryOptions")).Return(model.Artists{artist1}, nil).Once()
+
+		// Mock agent response with ID that won't be found, but MBID that will
+		agentSongs := []agents.Song{
+			{ID: "non-existent-id", Name: "Song One", MBID: "mbid-song-1"},
+		}
+		ag.On("GetArtistTopSongs", ctx, "artist-1", "Artist One", "mbid-artist-1", 1).Return(agentSongs, nil).Once()
+
+		// Mock ID lookup - returns empty (ID not found)
+		mediaFileRepo.On("GetAll", mock.AnythingOfType("model.QueryOptions")).Return(model.MediaFiles{}, nil).Once()
+		// Mock MBID lookup - finds the song
+		song1 := model.MediaFile{ID: "song-1", Title: "Song One", ArtistID: "artist-1", MbzRecordingID: "mbid-song-1"}
+		mediaFileRepo.On("GetAll", mock.AnythingOfType("model.QueryOptions")).Return(model.MediaFiles{song1}, nil).Once()
+
+		songs, err := p.TopSongs(ctx, "Artist One", 1)
+
+		Expect(err).ToNot(HaveOccurred())
+		Expect(songs).To(HaveLen(1))
+		Expect(songs[0].ID).To(Equal("song-1"))
+		artistRepo.AssertExpectations(GinkgoT())
+		ag.AssertExpectations(GinkgoT())
+		mediaFileRepo.AssertExpectations(GinkgoT())
+	})
 })