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/analysis/providers.ts

@@ -8,6 +8,8 @@ export interface AnalysisInput {
   mbid?: string | null;
   contributed_by: string;
   ollamaModel?: string;   // required when provider id is "ollama"
+  confirmedBpm?: number | null;        // from MusicBrainz tags or other reliable source
+  confirmedTimeSigNum?: number | null; // time signature numerator if confirmed
 }
 
 export interface ProviderInfo {