feat(tuning): scoring weights → DB-backed admin tuning lab

The recommendation scoring knobs move out of YAML (radio profile) and
out of the systemMixWeights hard-code (daily_mix profile) into
DB-backed settings with live effect (#1250) — the defaults-discovery
lab per decision #1247: the operator turns knobs to find good values,
which then get baked back into shipped defaults; end users and other
operators should never need the card.

- Migration 0040: recommendation_weight_profiles (radio / daily_mix,
  8 weight columns), taste_tuning singleton (engagement half-life +
  completion-curve points), recommendation_tuning_audit (one row per
  change with a {field, old, new} diff — the trend view's markers,
  #1251).
- internal/recsettings: boot reconcile seeds shipped defaults without
  clobbering tuned rows (coverart SettingsService pattern), validates
  patches (bounds, curve ordering), writes audit rows, and pushes
  daily_mix weights + taste config into package playlists. No-op
  patches write no audit row.
- playlists gains SetSystemMixWeights / SetTasteConfig swap points
  under a RWMutex — no signature threading through the producers; the
  scheduler's taste rebuild reads the pushed config.
- Radio reads its weight profile from the service per request; the 8
  weight fields leave config.RecommendationConfig (YAML keeps only
  RecentlyPlayedHours / RadioSize / RadioSizeMax).
- Admin API: GET/PATCH/reset under /api/admin/recommendation-tuning,
  echoing current + shipped values.
- Web: new admin Tuning tab — two weight profiles side by side, taste
  card, per-scope save (changed fields only) + reset, deviation dots
  against shipped defaults.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TsF3cNoKrqCYsU78cXC8U6

internal/playlists/you_might_like.go

@@ -127,11 +127,12 @@ func rollUpCandidates(
 	cands []recommendation.Candidate, userID pgtype.UUID, dateStr string, now time.Time,
 ) (albumIDs, artistIDs []pgtype.UUID) {
 	rng := rand.New(rand.NewSource(int64(userIDHash(userID, dateStr))))
+	weights := currentSystemMixWeights()
 	albumScores := map[pgtype.UUID][]float64{}
 	artistScores := map[pgtype.UUID][]float64{}
 	albumArtist := map[pgtype.UUID]pgtype.UUID{}
 	for _, c := range cands {
-		s := recommendation.Score(c.Inputs, systemMixWeights, now, rng.Float64)
+		s := recommendation.Score(c.Inputs, weights, now, rng.Float64)
 		if c.Track.AlbumID.Valid {
 			albumScores[c.Track.AlbumID] = append(albumScores[c.Track.AlbumID], s)
 			albumArtist[c.Track.AlbumID] = c.Track.ArtistID