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/analyze/route.ts

@@ -2,6 +2,8 @@ import { NextRequest, NextResponse } from "next/server";
 import { z } from "zod";
 import { getProvider, getAvailableProviders } from "@/lib/analysis/providers/registry";
 import { validateCTP } from "@/lib/ctp/validate";
+import { getSongByMbid, updateSongBpm } from "@/lib/db/client";
+import { lookupRecordingWithTags, extractBpmFromTags, extractTimeSigFromTags } from "@/lib/musicbrainz/client";
 
 // ─── Request schema ───────────────────────────────────────────────────────────
 
@@ -77,6 +79,43 @@ export async function POST(req: NextRequest) {
     }
   }
 
+  // ── BPM enrichment ────────────────────────────────────────────────────────
+  // If the song has an MBID, try to supply the AI with a confirmed BPM from
+  // MusicBrainz community tags. Best-effort: any failure is silently ignored.
+  let confirmedBpm: number | null = null;
+  let confirmedTimeSigNum: number | null = null;
+
+  if (mbid) {
+    // 1. Check the DB first — may already have a stored BPM from a prior lookup
+    try {
+      const storedSong = await getSongByMbid(mbid);
+      if (storedSong?.acousticbrainz_bpm) {
+        confirmedBpm = storedSong.acousticbrainz_bpm;
+        confirmedTimeSigNum = storedSong.acousticbrainz_time_sig_num ?? null;
+      }
+    } catch {
+      // ignore DB errors
+    }
+
+    // 2. If no stored BPM, fetch from MusicBrainz tags
+    if (!confirmedBpm) {
+      try {
+        const rec = await lookupRecordingWithTags(mbid);
+        const tagBpm = rec.tags ? extractBpmFromTags(rec.tags) : null;
+        const tagTimeSig = rec.tags ? extractTimeSigFromTags(rec.tags) : null;
+
+        if (tagBpm) {
+          confirmedBpm = tagBpm;
+          confirmedTimeSigNum = tagTimeSig;
+          // Persist for next time — fire-and-forget
+          updateSongBpm(mbid, tagBpm, tagTimeSig).catch(() => {});
+        }
+      } catch {
+        // MusicBrainz unavailable or rate-limited — proceed without confirmed BPM
+      }
+    }
+  }
+
   const input = {
     bpm,
     duration,
@@ -85,6 +124,8 @@ export async function POST(req: NextRequest) {
     mbid: mbid ?? null,
     contributed_by: contributed_by ?? "anonymous",
     ollamaModel,
+    confirmedBpm,
+    confirmedTimeSigNum,
   };
 
   let ctpDoc;