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/config/config.go

@@ -88,20 +88,14 @@ type EventsConfig struct {
 	SkipMaxDurationPlayedMs int     `yaml:"skip_max_duration_played_ms"`
 }
 
-// RecommendationConfig governs the M3 weighted-shuffle scoring (spec §6).
-// All weights are operator-tunable; defaults match the spec recommendations.
+// RecommendationConfig holds the radio path's operational knobs. The
+// scoring WEIGHTS moved to DB-backed admin settings (#1250,
+// internal/recsettings) — YAML is bootstrap-only; anything an operator
+// tunes lives in the UI with live effect.
 type RecommendationConfig struct {
-	BaseWeight          float64 `yaml:"base_weight"`
-	LikeBoost           float64 `yaml:"like_boost"`
-	RecencyWeight       float64 `yaml:"recency_weight"`
-	SkipPenalty         float64 `yaml:"skip_penalty"`
-	JitterMagnitude     float64 `yaml:"jitter_magnitude"`
-	ContextWeight       float64 `yaml:"context_weight"`
-	SimilarityWeight    float64 `yaml:"similarity_weight"`
-	TasteWeight         float64 `yaml:"taste_weight"`
-	RecentlyPlayedHours int     `yaml:"recently_played_hours"`
-	RadioSize           int     `yaml:"radio_size"`
-	RadioSizeMax        int     `yaml:"radio_size_max"`
+	RecentlyPlayedHours int `yaml:"recently_played_hours"`
+	RadioSize           int `yaml:"radio_size"`
+	RadioSizeMax        int `yaml:"radio_size_max"`
 }
 
 func Default() Config {
@@ -120,16 +114,6 @@ func Default() Config {
 			SkipMaxDurationPlayedMs: 30000,
 		},
 		Recommendation: RecommendationConfig{
-			BaseWeight:       1.0,
-			LikeBoost:        2.0,
-			RecencyWeight:    1.0,
-			SkipPenalty:      1.0,
-			JitterMagnitude:  0.1,
-			ContextWeight:    2.0,
-			SimilarityWeight: 2.0,
-			// Radio is seed-directed (the user picked a direction), so taste
-			// is a lighter nudge here than in the daily mixes (1.5).
-			TasteWeight:         1.0,
 			RecentlyPlayedHours: 1,
 			RadioSize:           50,
 			RadioSizeMax:        200,