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>

app/api/tracks/route.ts

@@ -2,7 +2,7 @@ import { NextRequest, NextResponse } from "next/server";
 import { z } from "zod";
 import { getTempoMapsForSong, getSongByMbid, insertTempoMap, upsertSong } from "@/lib/db/client";
 import { validateCTP } from "@/lib/ctp/validate";
-import { lookupRecording, formatArtistCredit, mbDurationToSeconds } from "@/lib/musicbrainz/client";
+import { getMusicBrainzRecording } from "@/lib/musicbrainz/client";
 
 // ─── GET /api/tracks?mbid=<uuid> ─────────────────────────────────────────────
 
@@ -61,14 +61,14 @@ export async function POST(req: NextRequest) {
 
   if (!existing) {
     try {
-      const rec = await lookupRecording(doc.metadata.mbid);
+      const mbRecord = await getMusicBrainzRecording(doc.metadata.mbid);
       await upsertSong({
         mbid: doc.metadata.mbid,
-        title: rec.title,
-        artist: formatArtistCredit(rec["artist-credit"]),
-        duration_seconds: mbDurationToSeconds(rec.length),
-        acousticbrainz_bpm: null,
-        acousticbrainz_time_sig_num: null,
+        title: mbRecord.title,
+        artist: mbRecord.artist,
+        duration_seconds: mbRecord.duration_seconds,
+        acousticbrainz_bpm: mbRecord.bpm,
+        acousticbrainz_time_sig_num: mbRecord.timeSigNum,
         source: "musicbrainz",
       });
     } catch {