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/api/radio.go

@@ -15,6 +15,7 @@ import (
 	"git.fabledsword.com/bvandeusen/minstrel/internal/apierror"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/recommendation"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/recsettings"
 )
 
 // RadioResponse is the body of GET /api/radio.
@@ -100,16 +101,9 @@ func (h *handlers) handleRadio(w http.ResponseWriter, r *http.Request) {
 		}
 	}
 
-	weights := recommendation.ScoringWeights{
-		BaseWeight:       h.recCfg.BaseWeight,
-		LikeBoost:        h.recCfg.LikeBoost,
-		RecencyWeight:    h.recCfg.RecencyWeight,
-		SkipPenalty:      h.recCfg.SkipPenalty,
-		JitterMagnitude:  h.recCfg.JitterMagnitude,
-		ContextWeight:    h.recCfg.ContextWeight,
-		SimilarityWeight: h.recCfg.SimilarityWeight,
-		TasteWeight:      h.recCfg.TasteWeight,
-	}
+	// Scoring weights come from the DB-backed tuning lab (#1250) —
+	// read per request so an admin change takes effect live.
+	weights := h.recSettings.Weights(recsettings.ScopeRadio)
 	picks := recommendation.Shuffle(candidates, weights, time.Now().UTC(), h.rng, limit-1)
 
 	out := make([]TrackRef, 0, len(picks)+1)