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/scheduler.go

@@ -236,7 +236,7 @@ func (s *Scheduler) runStartupCatchUp(ctx context.Context, users []dbq.ListActiv
 // build can read a fresh profile (phase-2 consumption); both steps are
 // best-effort and a failure in one is logged without blocking the other.
 func (s *Scheduler) rebuildUserDaily(ctx context.Context, userID pgtype.UUID, now time.Time) {
-	if err := taste.BuildTasteProfile(ctx, s.pool, s.logger, userID, taste.DefaultConfig()); err != nil {
+	if err := taste.BuildTasteProfile(ctx, s.pool, s.logger, userID, currentTasteConfig()); err != nil {
 		s.logger.Warn("scheduler: taste profile rebuild failed",
 			"user_id", uuidStringPL(userID), "err", err)
 	}

internal/playlists/system.go

@@ -15,6 +15,7 @@ import (
 	"math"
 	"math/rand"
 	"sort"
+	"sync"
 	"time"
 
 	"github.com/jackc/pgx/v5"
@@ -23,6 +24,7 @@ import (
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/recommendation"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/taste"
 )
 
 // seedArtistRow mirrors the sqlc-generated PickSeedArtistsRow shape.
@@ -175,24 +177,64 @@ func pickKindForMixTier(tier int32) string {
 
 const systemMixLength = 25
 
-// systemMixWeights are the fixed scoring weights used by the cron worker.
+// systemMixWeights are the scoring weights used by the daily builds.
 // JitterMagnitude is small (0.1) and combined with a userIDHash-seeded
 // RNG (see scoreAndSortCandidates) — same (user, day) produces same
 // scores within a day, but near-tied candidates reshuffle across days
 // so the playlist doesn't feel frozen.
-var systemMixWeights = recommendation.ScoringWeights{
-	BaseWeight:       1.0,
-	LikeBoost:        2.0,
-	RecencyWeight:    1.0,
-	SkipPenalty:      2.0,
-	JitterMagnitude:  0.1,
-	ContextWeight:    0.5,
-	SimilarityWeight: 1.5,
-	// Taste profile (#796 phase 2): the daily mixes are the primary
-	// taste-driven surface, so they lean on it. TasteMatchScore is in
-	// [-1,+1], so 1.5 makes a strong taste fit comparable to a like boost
-	// while passive avoidance (negative) gently demotes.
-	TasteWeight: 1.5,
+//
+// DB-tunable since #1250: the recsettings service pushes the current
+// daily_mix profile via SetSystemMixWeights at boot and on every admin
+// change (coverart Configure() pattern — no signature threading, live
+// effect without restart). The literal here is only the pre-push
+// value; shipped defaults live in recsettings.ShippedDailyMixWeights,
+// which must stay in sync with it.
+var (
+	systemTuningMu   sync.RWMutex
+	systemMixWeights = recommendation.ScoringWeights{
+		BaseWeight:       1.0,
+		LikeBoost:        2.0,
+		RecencyWeight:    1.0,
+		SkipPenalty:      2.0,
+		JitterMagnitude:  0.1,
+		ContextWeight:    0.5,
+		SimilarityWeight: 1.5,
+		// Taste profile (#796 phase 2): the daily mixes are the primary
+		// taste-driven surface, so they lean on it. TasteMatchScore is in
+		// [-1,+1], so 1.5 makes a strong taste fit comparable to a like boost
+		// while passive avoidance (negative) gently demotes.
+		TasteWeight: 1.5,
+	}
+	systemTasteConfig = taste.DefaultConfig()
+)
+
+// SetSystemMixWeights installs the current daily_mix scoring weights.
+// Called by the recsettings service at boot and on admin updates.
+func SetSystemMixWeights(w recommendation.ScoringWeights) {
+	systemTuningMu.Lock()
+	defer systemTuningMu.Unlock()
+	systemMixWeights = w
+}
+
+// SetTasteConfig installs the taste-profile build configuration
+// (half-life + engagement curve, #1250). Same push model as
+// SetSystemMixWeights.
+func SetTasteConfig(c taste.Config) {
+	systemTuningMu.Lock()
+	defer systemTuningMu.Unlock()
+	systemTasteConfig = c
+}
+
+func currentSystemMixWeights() recommendation.ScoringWeights {
+	systemTuningMu.RLock()
+	defer systemTuningMu.RUnlock()
+	return systemMixWeights
+}
+
+func currentTasteConfig() taste.Config {
+	systemTuningMu.RLock()
+	defer systemTuningMu.RUnlock()
+	return systemTasteConfig
 }
 
 // forYouHeadN is the number of top-scored tracks that anchor the For-You
@@ -355,9 +397,10 @@ func scoreAndSortCandidates(cands []recommendation.Candidate, userID pgtype.UUID
 	sort.SliceStable(ordered, func(i, j int) bool {
 		return uuidLessPL(ordered[i].Track.ID, ordered[j].Track.ID)
 	})
+	weights := currentSystemMixWeights()
 	pairs := make([]scored, len(ordered))
 	for i, c := range ordered {
-		pairs[i] = scored{c: c, score: recommendation.Score(c.Inputs, systemMixWeights, now, rng.Float64)}
+		pairs[i] = scored{c: c, score: recommendation.Score(c.Inputs, weights, now, rng.Float64)}
 	}
 	sort.SliceStable(pairs, func(i, j int) bool {
 		if pairs[i].score != pairs[j].score {
@@ -784,11 +827,12 @@ func pickTopN(cands []recommendation.Candidate, userID pgtype.UUID, dateStr stri
 		capped = capped[:n]
 	}
 	rng := rand.New(rand.NewSource(int64(userIDHash(userID, dateStr))))
+	weights := currentSystemMixWeights()
 	out := make([]rankedCandidate, len(capped))
 	for i, c := range capped {
 		out[i] = rankedCandidate{
 			TrackID: c.Track.ID,
-			Score:   recommendation.Score(c.Inputs, systemMixWeights, now, rng.Float64),
+			Score:   recommendation.Score(c.Inputs, weights, now, rng.Float64),
 		}
 	}
 	return out
@@ -815,6 +859,7 @@ func pickHeadAndTail(
 	sorted := scoreAndSortCandidates(cands, userID, dateStr, now)
 	capped := capCandidatesByAlbumAndArtist(sorted)
 	rng := rand.New(rand.NewSource(int64(userIDHash(userID, dateStr))))
+	weights := currentSystemMixWeights()
 
 	total := headN + tailN
 	if len(capped) <= total {
@@ -828,7 +873,7 @@ func pickHeadAndTail(
 		for i := 0; i < total; i++ {
 			out[i] = rankedCandidate{
 				TrackID:  capped[i].Track.ID,
-				Score:    recommendation.Score(capped[i].Inputs, systemMixWeights, now, rng.Float64),
+				Score:    recommendation.Score(capped[i].Inputs, weights, now, rng.Float64),
 				PickKind: pickKindTaste,
 			}
 		}
@@ -875,7 +920,7 @@ func pickHeadAndTail(
 		}
 		out[i] = rankedCandidate{
 			TrackID:  c.Track.ID,
-			Score:    recommendation.Score(c.Inputs, systemMixWeights, now, rng.Float64),
+			Score:    recommendation.Score(c.Inputs, weights, now, rng.Float64),
 			PickKind: kind,
 		}
 	}

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