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/api/admin_recommendation_tuning.go

@@ -67,15 +67,30 @@ func tasteRespFrom(t recsettings.TasteTuning) tasteTuningResp {
 	}
 }
 
+// discoverTuningResp is the Discover scope on the wire (#2377).
+type discoverTuningResp struct {
+	TagOverlapWeight float64 `json:"tag_overlap_weight"`
+	SnoozeDays       float64 `json:"snooze_days"`
+}
+
+func discoverRespFrom(d recsettings.DiscoverTuning) discoverTuningResp {
+	return discoverTuningResp{
+		TagOverlapWeight: d.TagOverlapWeight,
+		SnoozeDays:       d.SnoozeDays,
+	}
+}
+
 // tuningSnapshot is both the GET response and the post-mutation echo:
 // current values alongside shipped defaults so the card can mark
 // which knobs deviate.
 type tuningSnapshot struct {
 	Profiles map[string]weightsResp `json:"profiles"`
 	Taste    tasteTuningResp        `json:"taste"`
+	Discover discoverTuningResp     `json:"discover"`
 	Shipped  struct {
 		Profiles map[string]weightsResp `json:"profiles"`
 		Taste    tasteTuningResp        `json:"taste"`
+		Discover discoverTuningResp     `json:"discover"`
 	} `json:"shipped"`
 }
 
@@ -86,11 +101,13 @@ func (h *handlers) tuningSnapshot() tuningSnapshot {
 		recsettings.ScopeDailyMix: weightsRespFrom(h.recSettings.Weights(recsettings.ScopeDailyMix)),
 	}
 	out.Taste = tasteRespFrom(h.recSettings.Taste())
+	out.Discover = discoverRespFrom(h.recSettings.Discover())
 	out.Shipped.Profiles = map[string]weightsResp{
 		recsettings.ScopeRadio:    weightsRespFrom(recsettings.ShippedRadioWeights()),
 		recsettings.ScopeDailyMix: weightsRespFrom(recsettings.ShippedDailyMixWeights()),
 	}
 	out.Shipped.Taste = tasteRespFrom(recsettings.ShippedTasteTuning())
+	out.Shipped.Discover = discoverRespFrom(recsettings.ShippedDiscoverTuning())
 	return out
 }
 
@@ -120,9 +137,12 @@ func (h *handlers) handlePatchRecommendationTuning(w http.ResponseWriter, r *htt
 	}
 
 	var err error
-	if scope == recsettings.ScopeTaste {
+	switch scope {
+	case recsettings.ScopeTaste:
 		err = h.recSettings.UpdateTaste(r.Context(), body.Values)
-	} else {
+	case recsettings.ScopeDiscover:
+		err = h.recSettings.UpdateDiscover(r.Context(), body.Values)
+	default:
 		err = h.recSettings.UpdateProfile(r.Context(), scope, body.Values)
 	}
 	if err != nil {