feat: direct Spotify/Bandcamp links via MusicBrainz url-relations (task/odesli-direct-links)

The Odesli/Songlink API rejects artist URLs (405 UNSUPPORTED_URL — it is song/album
only), so the planned Odesli hop is not viable for artist linking. Pivoted to
MusicBrainz `inc=url-rels`, which returns the artist's official Bandcamp + Spotify
URLs directly, zero-cred:
- service/lib.js: relUrls() extracts direct bandcamp/spotify from MB relations;
  mbResult() prefers them, falls back to search URLs per-platform (replaces searchResult).
- service/resolver.js: viaMusicBrainz now does search -> url-rels enrichment (both
  throttled through one gate); resolveOne calls it directly.
- service/lib.test.js: +3 tests (relUrls + mbResult); 9 total, green.
- README: document the direct-link behavior.

Verified zero-cred end to end: "Cattle Decapitation" -> open.spotify.com/artist/... +
cattledecapitation.bandcamp.com/ (both direct, not search); nonsense -> null.

Source: task/odesli-direct-links (objective/linker-improvements); approach changed from
Odesli to MB url-rels after the API probe; goal (direct Bandcamp) achieved.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

service/lib.js

@@ -26,9 +26,20 @@ export const spotifyResult = (hit) => ({
   bandcamp: bandcampSearch(hit.name),
 });
 
-// Link object for a confirmed MusicBrainz hit (search urls — MB gives no platform ids).
-export const searchResult = (hit) => ({
-  name: hit.name,
-  spotify: spotifySearch(hit.name),
-  bandcamp: bandcampSearch(hit.name),
-});
+// Extract direct links from a MusicBrainz artist's url-relations (zero-cred: MB stores
+// official Bandcamp / streaming URLs). Either field may be null.
+export function relUrls(relations) {
+  const urls = (relations ?? []).map((r) => r && r.url && r.url.resource).filter(Boolean);
+  const find = (host) => urls.find((u) => u.includes(host)) ?? null;
+  return { bandcamp: find("bandcamp.com"), spotify: find("open.spotify.com") };
+}
+
+// Link object for a confirmed MusicBrainz hit: prefer the direct url-rel links, fall back
+// to search urls when MB has no relation for that platform.
+export function mbResult(hit, links) {
+  return {
+    name: hit.name,
+    spotify: links?.spotify ?? spotifySearch(hit.name),
+    bandcamp: links?.bandcamp ?? bandcampSearch(hit.name),
+  };
+}