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/resolver.js

@@ -10,7 +10,7 @@
 
 import { createServer } from "node:http";
 import { readFileSync } from "node:fs";
-import { norm, pickArtist, spotifyResult, searchResult } from "./lib.js";
+import { norm, pickArtist, spotifyResult, mbResult, relUrls } from "./lib.js";
 
 // --- minimal .env loader (no dependency) ------------------------------------
 try {
@@ -58,29 +58,41 @@ async function viaSpotify(name) {
   return hit ? spotifyResult(hit) : null;
 }
 
-// --- MusicBrainz (zero-cred fallback) ---------------------------------------
-// MusicBrainz asks for <=1 request/second, so issuance is serialized + spaced.
+// --- MusicBrainz (zero-cred path) -------------------------------------------
+// MusicBrainz asks for <=1 request/second, so every request is serialized + spaced.
 const sleep = (ms) => new Promise((r) => setTimeout(r, ms));
 let mbGate = Promise.resolve();
 let mbLast = 0;
-function mbThrottled(name) {
+function mbSchedule() {
   const gate = mbGate.then(async () => {
     const wait = 1100 - (Date.now() - mbLast);
     if (wait > 0) await sleep(wait);
     mbLast = Date.now();
   });
   mbGate = gate.catch(() => {}); // a failure never wedges the queue
-  return gate.then(() => viaMusicBrainz(name));
+  return gate;
+}
+async function mbFetchJson(url) {
+  await mbSchedule();
+  const r = await fetch(url, { headers: { "user-agent": UA, accept: "application/json" } });
+  if (!r.ok) throw new Error(`musicbrainz http ${r.status}`);
+  return r.json();
 }
 
 async function viaMusicBrainz(name) {
   const q = encodeURIComponent(`artist:"${name}"`);
-  const url = `https://musicbrainz.org/ws/2/artist?fmt=json&limit=5&query=${q}`;
-  const r = await fetch(url, { headers: { "user-agent": UA, accept: "application/json" } });
-  if (!r.ok) throw new Error(`musicbrainz http ${r.status}`);
-  const j = await r.json();
-  const hit = pickArtist(name, j.artists, { minScore: 90 });
-  return hit ? searchResult(hit) : null;
+  const search = await mbFetchJson(`https://musicbrainz.org/ws/2/artist?fmt=json&limit=5&query=${q}`);
+  const hit = pickArtist(name, search.artists, { minScore: 90 });
+  if (!hit) return null;
+  // Enrich with MB url-relations -> DIRECT bandcamp/spotify links (best-effort).
+  let links = null;
+  try {
+    const detail = await mbFetchJson(`https://musicbrainz.org/ws/2/artist/${hit.id}?inc=url-rels&fmt=json`);
+    links = relUrls(detail.relations);
+  } catch (e) {
+    console.error(`mb url-rels "${name}": ${e.message}`); // enrichment is best-effort
+  }
+  return mbResult(hit, links);
 }
 
 // --- resolve one candidate (cached) -----------------------------------------
@@ -89,7 +101,7 @@ async function resolveOne(name) {
   if (!key) return null;
   if (cache.has(key)) return cache.get(key);
   try {
-    const result = USE_SPOTIFY ? await viaSpotify(name) : await mbThrottled(name);
+    const result = USE_SPOTIFY ? await viaSpotify(name) : await viaMusicBrainz(name);
     cache.set(key, result); // cache hits AND confirmed misses
     return result;
   } catch (e) {