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_updateartistinfo_test.go

@@ -226,4 +226,88 @@ var _ = Describe("Provider - UpdateArtistInfo", func() {
 		Expect(updatedArtist.ID).To(Equal("ar-agent-fail"))
 		ag.AssertExpectations(GinkgoT())
 	})
+
+	It("matches similar artists by ID first when agent provides IDs", func() {
+		originalArtist := &model.Artist{
+			ID:   "ar-id-match",
+			Name: "ID Match Artist",
+		}
+		similarByID := model.Artist{ID: "ar-similar-by-id", Name: "Similar By ID", MbzArtistID: "mbid-similar"}
+		mockArtistRepo.SetData(model.Artists{*originalArtist, similarByID})
+
+		// Agent returns similar artist with ID (highest priority matching)
+		rawSimilar := []agents.Artist{
+			{ID: "ar-similar-by-id", Name: "Different Name", MBID: "different-mbid"},
+		}
+
+		ag.On("GetArtistMBID", ctx, "ar-id-match", "ID Match Artist").Return("", nil).Once()
+		ag.On("GetArtistImages", ctx, "ar-id-match", "ID Match Artist", mock.Anything).Return(nil, nil).Maybe()
+		ag.On("GetArtistBiography", ctx, "ar-id-match", "ID Match Artist", mock.Anything).Return("", nil).Maybe()
+		ag.On("GetArtistURL", ctx, "ar-id-match", "ID Match Artist", mock.Anything).Return("", nil).Maybe()
+		ag.On("GetSimilarArtists", ctx, "ar-id-match", "ID Match Artist", mock.Anything, 100).Return(rawSimilar, nil).Once()
+
+		updatedArtist, err := p.UpdateArtistInfo(ctx, "ar-id-match", 10, false)
+
+		Expect(err).NotTo(HaveOccurred())
+		Expect(updatedArtist.SimilarArtists).To(HaveLen(1))
+		// Should match by ID, not by name or MBID
+		Expect(updatedArtist.SimilarArtists[0].ID).To(Equal("ar-similar-by-id"))
+		Expect(updatedArtist.SimilarArtists[0].Name).To(Equal("Similar By ID"))
+	})
+
+	It("matches similar artists by MBID when ID is empty", func() {
+		originalArtist := &model.Artist{
+			ID:   "ar-mbid-match",
+			Name: "MBID Match Artist",
+		}
+		similarByMBID := model.Artist{ID: "ar-similar-by-mbid", Name: "Similar By MBID", MbzArtistID: "mbid-similar"}
+		mockArtistRepo.SetData(model.Artists{*originalArtist, similarByMBID})
+
+		// Agent returns similar artist with only MBID (no ID)
+		rawSimilar := []agents.Artist{
+			{Name: "Different Name", MBID: "mbid-similar"},
+		}
+
+		ag.On("GetArtistMBID", ctx, "ar-mbid-match", "MBID Match Artist").Return("", nil).Once()
+		ag.On("GetArtistImages", ctx, "ar-mbid-match", "MBID Match Artist", mock.Anything).Return(nil, nil).Maybe()
+		ag.On("GetArtistBiography", ctx, "ar-mbid-match", "MBID Match Artist", mock.Anything).Return("", nil).Maybe()
+		ag.On("GetArtistURL", ctx, "ar-mbid-match", "MBID Match Artist", mock.Anything).Return("", nil).Maybe()
+		ag.On("GetSimilarArtists", ctx, "ar-mbid-match", "MBID Match Artist", mock.Anything, 100).Return(rawSimilar, nil).Once()
+
+		updatedArtist, err := p.UpdateArtistInfo(ctx, "ar-mbid-match", 10, false)
+
+		Expect(err).NotTo(HaveOccurred())
+		Expect(updatedArtist.SimilarArtists).To(HaveLen(1))
+		// Should match by MBID since ID was empty
+		Expect(updatedArtist.SimilarArtists[0].ID).To(Equal("ar-similar-by-mbid"))
+		Expect(updatedArtist.SimilarArtists[0].Name).To(Equal("Similar By MBID"))
+	})
+
+	It("falls back to name matching when ID and MBID don't match", func() {
+		originalArtist := &model.Artist{
+			ID:   "ar-name-match",
+			Name: "Name Match Artist",
+		}
+		similarByName := model.Artist{ID: "ar-similar-by-name", Name: "Similar By Name"}
+		mockArtistRepo.SetData(model.Artists{*originalArtist, similarByName})
+
+		// Agent returns similar artist with non-matching ID and MBID
+		rawSimilar := []agents.Artist{
+			{ID: "non-existent-id", Name: "Similar By Name", MBID: "non-existent-mbid"},
+		}
+
+		ag.On("GetArtistMBID", ctx, "ar-name-match", "Name Match Artist").Return("", nil).Once()
+		ag.On("GetArtistImages", ctx, "ar-name-match", "Name Match Artist", mock.Anything).Return(nil, nil).Maybe()
+		ag.On("GetArtistBiography", ctx, "ar-name-match", "Name Match Artist", mock.Anything).Return("", nil).Maybe()
+		ag.On("GetArtistURL", ctx, "ar-name-match", "Name Match Artist", mock.Anything).Return("", nil).Maybe()
+		ag.On("GetSimilarArtists", ctx, "ar-name-match", "Name Match Artist", mock.Anything, 100).Return(rawSimilar, nil).Once()
+
+		updatedArtist, err := p.UpdateArtistInfo(ctx, "ar-name-match", 10, false)
+
+		Expect(err).NotTo(HaveOccurred())
+		Expect(updatedArtist.SimilarArtists).To(HaveLen(1))
+		// Should fall back to name matching since ID and MBID didn't match
+		Expect(updatedArtist.SimilarArtists[0].ID).To(Equal("ar-similar-by-name"))
+		Expect(updatedArtist.SimilarArtists[0].Name).To(Equal("Similar By Name"))
+	})
 })