feat: MusicBrainz BPM enrichment + improved AI prompts

- lookupRecordingWithTags, extractBpmFromTags, extractTimeSigFromTags, getMusicBrainzRecording added to MB client
- upsertSong preserves existing BPM via COALESCE on conflict
- updateSongBpm helper for async enrichment writes
- AnalysisInput gains confirmedBpm / confirmedTimeSigNum fields
- POST /api/analyze fetches confirmed BPM from DB then MB tags before generation
- All three AI providers use confirmedBpm as authoritative and build enriched userMessage
- POST /api/tracks auto-registration now fetches tags via getMusicBrainzRecording
- Updated User-Agent and MB client fallback URL to Gitea

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/musicbrainz/client.ts

@@ -10,7 +10,7 @@
 const MB_BASE = "https://musicbrainz.org/ws/2";
 const USER_AGENT =
   process.env.MUSICBRAINZ_USER_AGENT ??
-  "ClickTrack/0.1 ( https://github.com/your-org/clicktrack )";
+  "ClickTrack/0.1 ( https://git.avezzano.io/the_og/clicktrack )";
 
 // ─── Rate limiter ─────────────────────────────────────────────────────────────
 
@@ -60,6 +60,7 @@ export interface MBRecording {
   length?: number;      // duration in milliseconds
   "artist-credit": MBArtistCredit[];
   releases?: MBRelease[];
+  tags?: MBTag[];
   score?: number;       // search relevance (0–100)
 }
 
@@ -79,6 +80,11 @@ export interface MBRelease {
   status?: string;
 }
 
+export interface MBTag {
+  name: string;
+  count: number;
+}
+
 export interface MBSearchResult {
   created: string;
   count: number;
@@ -147,6 +153,100 @@ export async function lookupRecording(mbid: string): Promise<MBRecording> {
   return response.json() as Promise<MBRecording>;
 }
 
+/**
+ * Looks up a single recording by MBID, requesting artist-credits, releases,
+ * and community tags. Use this when you also want BPM or time-signature tags.
+ */
+export async function lookupRecordingWithTags(mbid: string): Promise<MBRecording> {
+  const params = new URLSearchParams({
+    inc: "artist-credits+releases+tags",
+    fmt: "json",
+  });
+
+  const url = `${MB_BASE}/recording/${encodeURIComponent(mbid)}?${params}`;
+  const response = await rateLimitedFetch(url);
+  return response.json() as Promise<MBRecording>;
+}
+
+/**
+ * Parses a MusicBrainz tag list and extracts a BPM value if present.
+ *
+ * MusicBrainz users tag recordings with strings like:
+ *   "bpm: 174", "174 bpm", "bpm:174", "tempo: 174"
+ *
+ * Returns the most-voted BPM value (highest tag count), or null if none found.
+ */
+export function extractBpmFromTags(tags: MBTag[]): number | null {
+  const bpmPattern = /(?:bpm|tempo)\s*:?\s*(\d{2,3})|(\d{2,3})\s*bpm/i;
+
+  let bestBpm: number | null = null;
+  let bestCount = 0;
+
+  for (const tag of tags) {
+    const match = bpmPattern.exec(tag.name);
+    if (match) {
+      const value = parseInt(match[1] ?? match[2], 10);
+      if (value >= 20 && value <= 400 && tag.count > bestCount) {
+        bestBpm = value;
+        bestCount = tag.count;
+      }
+    }
+  }
+
+  return bestBpm;
+}
+
+/**
+ * Parses a MusicBrainz tag list and extracts a time signature numerator if present.
+ *
+ * Users tag recordings like "3/4", "5/4", "6/8", "time signature: 3/4".
+ * We only store the numerator since that's what the CTP denominator-agnostic
+ * count-in uses.
+ *
+ * Returns the most-voted numerator, or null if none found.
+ */
+export function extractTimeSigFromTags(tags: MBTag[]): number | null {
+  const timeSigPattern = /(?:time\s*signature\s*:?\s*)?(\d{1,2})\/(\d{1,2})/i;
+
+  let bestNum: number | null = null;
+  let bestCount = 0;
+
+  for (const tag of tags) {
+    const match = timeSigPattern.exec(tag.name);
+    if (match) {
+      const numerator = parseInt(match[1], 10);
+      if (numerator >= 1 && numerator <= 32 && tag.count > bestCount) {
+        bestNum = numerator;
+        bestCount = tag.count;
+      }
+    }
+  }
+
+  return bestNum;
+}
+
+/**
+ * Returns a normalised recording object suitable for passing to `upsertSong`.
+ * Includes BPM and time-signature from community tags when available.
+ */
+export async function getMusicBrainzRecording(mbid: string): Promise<{
+  title: string;
+  artist: string;
+  duration_seconds: number | null;
+  bpm: number | null;
+  timeSigNum: number | null;
+}> {
+  const rec = await lookupRecordingWithTags(mbid);
+
+  return {
+    title: rec.title,
+    artist: formatArtistCredit(rec["artist-credit"]),
+    duration_seconds: mbDurationToSeconds(rec.length),
+    bpm: rec.tags ? extractBpmFromTags(rec.tags) : null,
+    timeSigNum: rec.tags ? extractTimeSigFromTags(rec.tags) : null,
+  };
+}
+
 /**
  * Convenience function: searches MusicBrainz and returns results normalised
  * for storage in the `songs` table.