Add optional MusicBrainz/Cover Art Archive metadata enrichment

getArtistInfo/2 and getAlbumInfo/2 now return a real MusicBrainz
disambiguation/cover-art lookup for entities that carry a locally
tagged MusicBrainz ID, gated behind the new
VIBESONIC_ENABLE_EXTERNAL_METADATA flag (default off). Responses are
cached in the existing external_metadata_cache table for 30 days to
avoid hammering the free MusicBrainz/CAA services.

VibeSonic still works completely offline: with the flag off, or when
an entity has no tagged MusicBrainz ID, or on any network/lookup
failure, the handlers fall back to local-only fields exactly as
before — nothing is ever fabricated, and a failed external lookup
never fails the request.

internal/metadataenrich is a small, independently-testable HTTP client
for the two real APIs (base URLs overridable for tests, exercised
against httptest servers rather than the live network).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014r98AtcBbevpscWLsfwVvv

internal/metadataenrich/client_test.go

@@ -0,0 +1,119 @@
+package metadataenrich
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+func TestFetchArtistParsesRealFields(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path != "/artist/abc-123" {
+			t.Errorf("unexpected path %q", r.URL.Path)
+		}
+		if r.Header.Get("User-Agent") == "" {
+			t.Error("MusicBrainz requires a non-empty User-Agent")
+		}
+		json.NewEncoder(w).Encode(map[string]any{
+			"disambiguation": "English rock band",
+			"tags":           []map[string]string{{"name": "rock"}, {"name": "alternative"}},
+		})
+	}))
+	defer srv.Close()
+
+	c := &Client{MusicBrainzBaseURL: srv.URL}
+	data, err := c.FetchArtist(context.Background(), "abc-123")
+	if err != nil {
+		t.Fatalf("FetchArtist: %v", err)
+	}
+	if data.Disambiguation != "English rock band" {
+		t.Errorf("disambiguation = %q", data.Disambiguation)
+	}
+	if len(data.Genres) != 2 || data.Genres[0] != "rock" {
+		t.Errorf("genres = %v", data.Genres)
+	}
+}
+
+func TestFetchArtistHTTPError(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusNotFound)
+	}))
+	defer srv.Close()
+
+	c := &Client{MusicBrainzBaseURL: srv.URL}
+	if _, err := c.FetchArtist(context.Background(), "does-not-exist"); err == nil {
+		t.Fatal("expected an error for a 404 from MusicBrainz, got nil")
+	}
+}
+
+func TestFetchReleaseCoverArtParsesFrontImage(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		json.NewEncoder(w).Encode(map[string]any{
+			"images": []map[string]any{
+				{"front": false, "image": "http://example.com/back.jpg"},
+				{
+					"front": true,
+					"image": "http://example.com/full.jpg",
+					"thumbnails": map[string]string{
+						"small": "http://example.com/small.jpg",
+						"large": "http://example.com/large.jpg",
+					},
+				},
+			},
+		})
+	}))
+	defer srv.Close()
+
+	c := &Client{CoverArtArchiveBaseURL: srv.URL}
+	art, err := c.FetchReleaseCoverArt(context.Background(), "release-1")
+	if err != nil {
+		t.Fatalf("FetchReleaseCoverArt: %v", err)
+	}
+	if art == nil {
+		t.Fatal("expected cover art, got nil")
+	}
+	if art.LargeURL != "http://example.com/full.jpg" {
+		t.Errorf("LargeURL = %q", art.LargeURL)
+	}
+	if art.SmallURL != "http://example.com/small.jpg" || art.MediumURL != "http://example.com/large.jpg" {
+		t.Errorf("art = %+v", art)
+	}
+}
+
+func TestFetchReleaseCoverArtNotFoundReturnsNilNotError(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusNotFound)
+	}))
+	defer srv.Close()
+
+	c := &Client{CoverArtArchiveBaseURL: srv.URL}
+	art, err := c.FetchReleaseCoverArt(context.Background(), "no-art")
+	if err != nil {
+		t.Fatalf("expected nil error for a release with no registered cover art, got %v", err)
+	}
+	if art != nil {
+		t.Errorf("expected nil CoverArt, got %+v", art)
+	}
+}
+
+func TestFetchReleaseCoverArtNoFrontImage(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		json.NewEncoder(w).Encode(map[string]any{
+			"images": []map[string]any{
+				{"front": false, "image": "http://example.com/back.jpg"},
+			},
+		})
+	}))
+	defer srv.Close()
+
+	c := &Client{CoverArtArchiveBaseURL: srv.URL}
+	art, err := c.FetchReleaseCoverArt(context.Background(), "back-only")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if art != nil {
+		t.Errorf("expected nil when no image is marked front, got %+v (must never substitute a back cover)", art)
+	}
+}