feat(discover): rank suggestions by taste-tag overlap — #2377 (server)

The payoff slice. Until now a candidate's only claim on a slot was "some
artist you play is adjacent to it in a similarity graph" — a fact that says
nothing about whether the music sounds like anything you like. Now the
candidate's own folksonomy tags (cached by slice 5) are compared against
the user's taste-profile tags, so the deck ranks on taste and can say WHY.

The blend is MULTIPLICATIVE — score × (1 + weight × overlap) — and that
choice carries the whole safety argument:

  - An untagged candidate has overlap 0, so its score is EXACTLY unchanged.
    Tag coverage is permanently partial (#2376); it must cost a candidate
    nothing, not sink it (rule #131).
  - Nothing can leapfrog on tags alone. An additive term with a large
    weight would let a near-zero-similarity artist outrank a strong match
    for sharing one popular tag, which reads as noise.
  - Weight 0 restores pure similarity order bit-for-bit, so the operator's
    knob has a real off position.

overlap = Σ(shared) candWeight × normalizedTasteWeight ÷ Σ(all) candWeight.
Normalizing the taste side by the user's strongest tag makes the score
comparable across users (taste weights accumulate with listening, so a
heavy listener's raw numbers dwarf a new user's while meaning the same
thing). Dividing by the candidate's own mass makes it comparable across
candidates, so a densely-tagged artist can't win on tag count alone.

Applied to the whole over-fetched pool BEFORE selectSuggestions, so the
rotation and diversity rules operate on blended scores — boosting only the
twelve already chosen by similarity would leave the re-ranking undone.

A query failure is returned, NOT degraded past. Graceful degradation is
for expected absence (no taste profile, no cached tags) and both are
handled explicitly as empty inputs; swallowing a real error would hide a
broken DB behind a subtly worse ranking that nothing reports.

Migration 0051 adds a FOURTH tuning scope rather than columns on
taste_tuning, because snooze_days lives here too and a snooze must never
be read as taste signal (#2374) — filing it under 'taste' would put it one
careless join from the leak that design forbids. Expanding
recommendation_tuning_audit's CHECK is in the same migration per rule #36,
and a test asserts the audit row lands, which is what would catch its
absence.

snooze_days moves out of a Go constant onto the tuning card (rule #25),
closing the deferral from #2374.

Tag-overlap tests use deliberately SKEWED fixtures: an evenly-matching pool
cannot exercise a re-ranking, since every candidate gets the same
multiplier and the order is unchanged whether the blend works or not.

Admin UI + client attribution follow in this batch — rule #27.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

internal/recsettings/service.go

@@ -35,6 +35,11 @@ const (
 	ScopeRadio    = "radio"
 	ScopeDailyMix = "daily_mix"
 	ScopeTaste    = "taste"
+	// ScopeDiscover is the Discover request surface (#2377). Its own scope
+	// rather than columns on taste: SnoozeDays lives here, and a snooze must
+	// never be read as taste signal (#2374) — filing it under taste would put
+	// it one careless join from the leak that design forbids.
+	ScopeDiscover = "discover"
 )
 
 // TasteTuning is the tunable subset of taste.Config: the engagement
@@ -87,6 +92,33 @@ func ShippedDailyMixWeights() recommendation.ScoringWeights {
 	}
 }
 
+// DiscoverTuning is the tunable set for the Discover request surface (#2377).
+type DiscoverTuning struct {
+	// TagOverlapWeight scales the taste-tag term: score × (1 + w × overlap).
+	// 0 disables it and restores pure similarity ranking.
+	TagOverlapWeight float64
+	// SnoozeDays is the default "not right now" duration (#2374).
+	SnoozeDays float64
+}
+
+// ShippedDiscoverTuning are the shipped Discover defaults.
+//
+// TagOverlapWeight 1.0 lets a perfect tag match at most double a candidate's
+// similarity score — enough to reorder the deck meaningfully, not enough for a
+// popular-tag coincidence to beat a genuinely strong similarity match. It is a
+// starting point for the tuning lab, not a tuned value: the honest way to pick
+// it is the metrics trend view after some real use.
+//
+// SnoozeDays 90 matches the operator's approved shape: long enough that a
+// parked suggestion stops nagging, short enough that a taste shift brings it
+// back on its own.
+func ShippedDiscoverTuning() DiscoverTuning {
+	return DiscoverTuning{
+		TagOverlapWeight: 1.0,
+		SnoozeDays:       90,
+	}
+}
+
 // ShippedTasteTuning mirrors taste.DefaultConfig's tunable subset.
 func ShippedTasteTuning() TasteTuning {
 	d := taste.DefaultConfig()
@@ -110,6 +142,7 @@ type Service struct {
 	mu       sync.RWMutex
 	profiles map[string]recommendation.ScoringWeights
 	taste    TasteTuning
+	discover DiscoverTuning
 }
 
 // New boots the service: seeds shipped defaults for missing rows,
@@ -151,6 +184,13 @@ func (s *Service) reconcile(ctx context.Context) error {
 	}); err != nil {
 		return fmt.Errorf("seed taste tuning: %w", err)
 	}
+	sd := ShippedDiscoverTuning()
+	if err := q.UpsertDiscoverTuningDefaults(ctx, dbq.UpsertDiscoverTuningDefaultsParams{
+		TagOverlapWeight: sd.TagOverlapWeight,
+		SnoozeDays:       sd.SnoozeDays,
+	}); err != nil {
+		return fmt.Errorf("seed discover tuning: %w", err)
+	}
 
 	rows, err := q.ListWeightProfiles(ctx)
 	if err != nil {
@@ -160,6 +200,10 @@ func (s *Service) reconcile(ctx context.Context) error {
 	if err != nil {
 		return fmt.Errorf("get taste tuning: %w", err)
 	}
+	dt, err := q.GetDiscoverTuning(ctx)
+	if err != nil {
+		return fmt.Errorf("get discover tuning: %w", err)
+	}
 
 	s.mu.Lock()
 	s.profiles = map[string]recommendation.ScoringWeights{}
@@ -175,6 +219,10 @@ func (s *Service) reconcile(ctx context.Context) error {
 		EraScale:           tt.EraScale,
 		MoodScale:          tt.MoodScale,
 	}
+	s.discover = DiscoverTuning{
+		TagOverlapWeight: dt.TagOverlapWeight,
+		SnoozeDays:       dt.SnoozeDays,
+	}
 	s.mu.Unlock()
 
 	s.push()
@@ -208,6 +256,15 @@ func (s *Service) Taste() TasteTuning {
 	return s.taste
 }
 
+// Discover returns the cached Discover-tuning values. Read per request by the
+// suggestions handler, so an admin change takes effect on the next refresh
+// with no restart (rule #25).
+func (s *Service) Discover() DiscoverTuning {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	return s.discover
+}
+
 // TasteConfig assembles the full taste.Config the profile builder
 // consumes: shipped non-tunable knobs (like bonuses, floors, caps)
 // plus the tuned half-life and curve. WindowDays scales with the
@@ -267,6 +324,19 @@ func (s *Service) UpdateTaste(ctx context.Context, patch map[string]float64) err
 	return s.persistTaste(ctx, next, "update", changes)
 }
 
+// UpdateDiscover applies a partial update to the Discover tuning singleton.
+func (s *Service) UpdateDiscover(ctx context.Context, patch map[string]float64) error {
+	current := s.Discover()
+	next, changes, err := applyDiscoverPatch(current, patch)
+	if err != nil {
+		return err
+	}
+	if len(changes) == 0 {
+		return nil
+	}
+	return s.persistDiscover(ctx, next, "update", changes)
+}
+
 // Reset restores a scope to its shipped defaults, with one audit row
 // carrying the full diff. A scope already at defaults is a no-op.
 func (s *Service) Reset(ctx context.Context, scope string) error {
@@ -288,6 +358,13 @@ func (s *Service) Reset(ctx context.Context, scope string) error {
 			return nil
 		}
 		return s.persistTaste(ctx, shipped, "reset", changes)
+	case ScopeDiscover:
+		shipped := ShippedDiscoverTuning()
+		changes := diffDiscover(s.Discover(), shipped)
+		if len(changes) == 0 {
+			return nil
+		}
+		return s.persistDiscover(ctx, shipped, "reset", changes)
 	default:
 		return fmt.Errorf("%w: %q", ErrUnknownScope, scope)
 	}
@@ -338,6 +415,28 @@ func (s *Service) persistTaste(
 	return nil
 }
 
+// persistDiscover writes the discover row + audit entry and refreshes the
+// cache. No push(): unlike taste and daily_mix, nothing precomputes from these
+// — the suggestions handler reads Discover() per request.
+func (s *Service) persistDiscover(
+	ctx context.Context, d DiscoverTuning, action string, changes []fieldChange,
+) error {
+	q := dbq.New(s.pool)
+	if _, err := q.UpdateDiscoverTuning(ctx, dbq.UpdateDiscoverTuningParams{
+		TagOverlapWeight: d.TagOverlapWeight,
+		SnoozeDays:       d.SnoozeDays,
+	}); err != nil {
+		return fmt.Errorf("update discover tuning: %w", err)
+	}
+	if err := s.audit(ctx, q, ScopeDiscover, action, changes); err != nil {
+		return err
+	}
+	s.mu.Lock()
+	s.discover = d
+	s.mu.Unlock()
+	return nil
+}
+
 // audit writes one recommendation_tuning_audit row. Changes are
 // sorted by field so rows are deterministic and diff-friendly.
 func (s *Service) audit(