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/anthropic.ts

@@ -81,7 +81,9 @@ export const CTP_SCHEMA = {
 export const SYSTEM_PROMPT = `\
 You are an expert music producer and session musician assisting cover bands with click tracks.
 
-You will receive automated BPM detection results for a song and must generate a CTP (Click Track Protocol) document describing the song's full tempo map.
+You will receive information about a song and must generate a CTP (Click Track Protocol) document describing the song's full tempo map.
+
+**Use your training knowledge of the specific song.** If you recognise the title and artist, use what you know about its actual structure: section names, bar counts, time signature, and any tempo changes (ritardando, double-time, key change with tempo shift). Your training data is a valuable source — do not ignore it in favour of generic guesses.
 
 CTP rules:
 - "version" must be "1.0"
@@ -96,10 +98,15 @@ CTP rules:
 Guidelines for section layout:
 - Use typical pop/rock section names: Intro, Verse, Pre-Chorus, Chorus, Bridge, Outro
 - Estimate bar counts based on song duration and BPM (bars = duration_seconds × BPM / 60 / beats_per_bar)
-- Most songs are 4/4; note any unusual meters if you know the song
-- If you know the song has a tempo change (ritardando, double-time feel, key change with tempo shift), model it with a ramp or step section
+- Most songs are 4/4; use 3/4, 6/8, etc. if you know the song uses that meter
+- If the song has a tempo change (ritardando, double-time feel, key change with tempo shift), model it with a ramp or step section
 - If unsure about sections, use a single constant-tempo section covering the whole song
-- Use the detected BPM as the primary tempo — do not invent a different BPM unless the song is well-known to have a different tempo
+
+**BPM authority:** When a "Confirmed BPM" is provided in the user message, it comes from
+MusicBrainz community tags or a reliable reference — treat it as ground truth and use it
+for all sections unless you know the song has a significant tempo change. Do not average it
+with the detected BPM or discard it. When only a "Detected BPM" is provided, use it as a
+starting point but apply your knowledge of the song if you recognise it.
 
 The output is a draft for human review. Add reasonable section structure based on the song's typical arrangement.`;
 
@@ -118,9 +125,18 @@ export const anthropicProvider: AnalysisProvider = {
   },
 
   async generateCTP(input: AnalysisInput): Promise<CTPDocument> {
-    const { bpm, duration, title, artist, mbid, contributed_by } = input;
+    const { bpm, duration, title, artist, mbid, contributed_by, confirmedBpm, confirmedTimeSigNum } = input;
     const model = process.env.ANTHROPIC_MODEL ?? "claude-opus-4-6";
-    const approxBars = Math.round((duration * bpm) / 60 / 4);
+    const effectiveBpm = confirmedBpm ?? bpm;
+    const approxBars = Math.round((duration * effectiveBpm) / 60 / 4);
+
+    const bpmLine = confirmedBpm
+      ? `Confirmed BPM (from MusicBrainz community tags — treat as authoritative): ${confirmedBpm}\nDetected BPM (audio analysis): ${bpm}`
+      : `Detected BPM (audio analysis): ${bpm}`;
+
+    const timeSigHint = confirmedTimeSigNum
+      ? `\nConfirmed time signature numerator: ${confirmedTimeSigNum}`
+      : "";
 
     const userMessage = `\
 Generate a CTP document for the following song:
@@ -128,11 +144,11 @@ Generate a CTP document for the following song:
 Title: ${title ?? "Unknown Title"}
 Artist: ${artist ?? "Unknown Artist"}
 MusicBrainz ID: ${mbid ?? "unknown"}
-Detected BPM: ${bpm}
+${bpmLine}${timeSigHint}
 Duration: ${duration.toFixed(1)} seconds (~${approxBars} bars at 4/4)
 Contributed by: ${contributed_by}
 
-Create a plausible section layout for this song. If this is a well-known song, use your knowledge of its actual arrangement. If not, use a sensible generic structure.`;
+If you recognise this song, use your training knowledge of its actual arrangement — section names, bar counts, time signature, and any tempo changes. If you do not recognise it, use a sensible generic structure based on the BPM and duration above.`;
 
     // thinking and output_config are not yet in the SDK type definitions;
     // cast through the base param type to avoid type errors.