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/recommendation/suggestions_integration_test.go

@@ -147,7 +147,7 @@ func TestSuggestArtists_LikesAndPlaysContributeToScore(t *testing.T) {
 	seedUnmatched(t, pool, seedA.ID, "out-mbid", "Outsider", 0.9)
 	seedUnmatched(t, pool, seedB.ID, "out-mbid", "Outsider", 0.5)
 
-	out, err := SuggestArtists(context.Background(), pool, user.ID, 30, 12)
+	out, err := SuggestArtists(context.Background(), pool, user.ID, 30, 12, 0)
 	if err != nil {
 		t.Fatalf("SuggestArtists: %v", err)
 	}
@@ -174,7 +174,7 @@ func TestSuggestArtists_Top12Cap(t *testing.T) {
 	for i := 0; i < 30; i++ {
 		seedUnmatched(t, pool, seed.ID, fmt.Sprintf("mbid-%02d", i), fmt.Sprintf("Artist %02d", i), 0.99-float64(i)*0.01)
 	}
-	out, err := SuggestArtists(context.Background(), pool, user.ID, 30, 12)
+	out, err := SuggestArtists(context.Background(), pool, user.ID, 30, 12, 0)
 	if err != nil {
 		t.Fatalf("SuggestArtists: %v", err)
 	}
@@ -195,7 +195,7 @@ func TestSuggestArtists_AttributionTopThree(t *testing.T) {
 		likeArtist(t, pool, user.ID, seeds[i].ID)
 		seedUnmatched(t, pool, seeds[i].ID, "shared-mbid", "Shared", 0.9-float64(i)*0.1)
 	}
-	out, err := SuggestArtists(context.Background(), pool, user.ID, 30, 12)
+	out, err := SuggestArtists(context.Background(), pool, user.ID, 30, 12, 0)
 	if err != nil {
 		t.Fatalf("SuggestArtists: %v", err)
 	}
@@ -227,7 +227,7 @@ func TestSuggestArtists_RecencyDecayDownweightsOldPlays(t *testing.T) {
 	seedUnmatched(t, pool, recentSeed.ID, "cand", "Cand", 0.5)
 	seedUnmatched(t, pool, oldSeed.ID, "cand", "Cand", 0.5)
 
-	out, err := SuggestArtists(context.Background(), pool, user.ID, 30, 12)
+	out, err := SuggestArtists(context.Background(), pool, user.ID, 30, 12, 0)
 	if err != nil {
 		t.Fatalf("SuggestArtists: %v", err)
 	}
@@ -255,7 +255,7 @@ func TestSuggestArtists_FiltersInLibraryCandidates(t *testing.T) {
 	seedArtist(t, pool, "InLib", inLibMBID)
 	seedUnmatched(t, pool, seed.ID, inLibMBID, "InLib", 0.9)
 
-	out, err := SuggestArtists(context.Background(), pool, user.ID, 30, 12)
+	out, err := SuggestArtists(context.Background(), pool, user.ID, 30, 12, 0)
 	if err != nil {
 		t.Fatalf("SuggestArtists: %v", err)
 	}
@@ -279,7 +279,7 @@ func TestSuggestArtists_FiltersAlreadyRequested(t *testing.T) {
 		t.Fatalf("CreateLidarrRequest: %v", err)
 	}
 
-	out, err := SuggestArtists(context.Background(), pool, user.ID, 30, 12)
+	out, err := SuggestArtists(context.Background(), pool, user.ID, 30, 12, 0)
 	if err != nil {
 		t.Fatalf("SuggestArtists: %v", err)
 	}
@@ -310,7 +310,7 @@ func TestSuggestArtists_RejectedRequestStillShown(t *testing.T) {
 		t.Fatalf("RejectLidarrRequest: %v", err)
 	}
 
-	out, err := SuggestArtists(context.Background(), pool, user.ID, 30, 12)
+	out, err := SuggestArtists(context.Background(), pool, user.ID, 30, 12, 0)
 	if err != nil {
 		t.Fatalf("SuggestArtists: %v", err)
 	}
@@ -322,7 +322,7 @@ func TestSuggestArtists_RejectedRequestStillShown(t *testing.T) {
 func TestSuggestArtists_EmptyForNewUser(t *testing.T) {
 	pool := newPool(t)
 	user := seedUser(t, pool, "newbie")
-	out, err := SuggestArtists(context.Background(), pool, user.ID, 30, 12)
+	out, err := SuggestArtists(context.Background(), pool, user.ID, 30, 12, 0)
 	if err != nil {
 		t.Fatalf("SuggestArtists: %v", err)
 	}
@@ -374,7 +374,7 @@ func TestSuggestArtists_TasteProfileWeightSeedsTier1(t *testing.T) {
 	setTasteWeight(t, pool, user.ID, seed.ID, 4.0)
 	seedUnmatched(t, pool, seed.ID, "out-mbid", "Outsider", 0.9)
 
-	out, err := SuggestArtists(context.Background(), pool, user.ID, 30, 12)
+	out, err := SuggestArtists(context.Background(), pool, user.ID, 30, 12, 0)
 	if err != nil {
 		t.Fatalf("SuggestArtists: %v", err)
 	}
@@ -408,7 +408,7 @@ func TestSuggestArtists_TasteProfileSupersedesRawPlays(t *testing.T) {
 	seedUnmatched(t, pool, kept.ID, "kept-cand", "Kept Candidate", 0.9)
 	seedUnmatched(t, pool, dropped.ID, "dropped-cand", "Dropped Candidate", 0.9)
 
-	out, err := SuggestArtists(context.Background(), pool, user.ID, 30, 12)
+	out, err := SuggestArtists(context.Background(), pool, user.ID, 30, 12, 0)
 	if err != nil {
 		t.Fatalf("SuggestArtists: %v", err)
 	}
@@ -435,7 +435,7 @@ func TestSuggestArtists_NonPositiveTasteWeightDoesNotSeed(t *testing.T) {
 	seedUnmatched(t, pool, positive.ID, "pos-cand", "Positive Candidate", 0.9)
 	seedUnmatched(t, pool, abandoned.ID, "neg-cand", "Negative Candidate", 0.9)
 
-	out, err := SuggestArtists(context.Background(), pool, user.ID, 30, 12)
+	out, err := SuggestArtists(context.Background(), pool, user.ID, 30, 12, 0)
 	if err != nil {
 		t.Fatalf("SuggestArtists: %v", err)
 	}
@@ -464,7 +464,7 @@ func TestSuggestArtists_SkippedPlaysDoNotSeedTier2(t *testing.T) {
 	}
 	seedUnmatched(t, pool, skipped.ID, "skip-cand", "Skip Candidate", 0.9)
 
-	out, err := SuggestArtists(context.Background(), pool, user.ID, 30, 12)
+	out, err := SuggestArtists(context.Background(), pool, user.ID, 30, 12, 0)
 	if err != nil {
 		t.Fatalf("SuggestArtists: %v", err)
 	}
@@ -518,7 +518,7 @@ func TestSuggestArtists_ActiveSnoozeHidesCandidate(t *testing.T) {
 		t.Fatalf("SnoozeSuggestion: %v", err)
 	}
 
-	out, err := SuggestArtists(context.Background(), pool, user.ID, 30, 12)
+	out, err := SuggestArtists(context.Background(), pool, user.ID, 30, 12, 0)
 	if err != nil {
 		t.Fatalf("SuggestArtists: %v", err)
 	}
@@ -534,7 +534,7 @@ func TestSuggestArtists_ExpiredSnoozeShowsCandidateAgain(t *testing.T) {
 	seedOneCandidate(t, pool, user.ID, "expired-mbid", "Returning Artist")
 	snoozeUntil(t, pool, user.ID, "expired-mbid", "Returning Artist", time.Now().Add(-time.Hour))
 
-	out, err := SuggestArtists(context.Background(), pool, user.ID, 30, 12)
+	out, err := SuggestArtists(context.Background(), pool, user.ID, 30, 12, 0)
 	if err != nil {
 		t.Fatalf("SuggestArtists: %v", err)
 	}
@@ -560,14 +560,14 @@ func TestSuggestArtists_SnoozeIsPerUser(t *testing.T) {
 
 	snoozeUntil(t, pool, alice.ID, "shared-mbid", "Shared Candidate", time.Now().Add(24*time.Hour))
 
-	aliceOut, err := SuggestArtists(context.Background(), pool, alice.ID, 30, 12)
+	aliceOut, err := SuggestArtists(context.Background(), pool, alice.ID, 30, 12, 0)
 	if err != nil {
 		t.Fatalf("SuggestArtists(alice): %v", err)
 	}
 	if len(aliceOut) != 0 {
 		t.Errorf("alice len = %d, want 0 (she snoozed it)", len(aliceOut))
 	}
-	bobOut, err := SuggestArtists(context.Background(), pool, bob.ID, 30, 12)
+	bobOut, err := SuggestArtists(context.Background(), pool, bob.ID, 30, 12, 0)
 	if err != nil {
 		t.Fatalf("SuggestArtists(bob): %v", err)
 	}
@@ -604,7 +604,7 @@ func TestUnsnoozeSuggestion_RestoresImmediately(t *testing.T) {
 		t.Errorf("repeat rows = %d, want 0", rows)
 	}
 
-	out, err := SuggestArtists(context.Background(), pool, user.ID, 30, 12)
+	out, err := SuggestArtists(context.Background(), pool, user.ID, 30, 12, 0)
 	if err != nil {
 		t.Fatalf("SuggestArtists: %v", err)
 	}