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.go

@@ -31,6 +31,15 @@ type ArtistSuggestion struct {
 	Name        string
 	Score       float64
 	Attribution []SeedContribution
+	// MatchedTags are the candidate's own tags that overlap the user's taste
+	// profile, strongest first (max 3) — the "matches: shoegaze, melancholic"
+	// explanation (#2377). Empty when the candidate has no cached tags, which
+	// is common and not an error: coverage is permanently partial (#2376).
+	MatchedTags []string
+	// TagOverlap is the [0,1] share of the candidate's tag mass the user likes.
+	// Exposed for the admin tuning lab — seeing the term's actual distribution
+	// is how the operator picks a weight rather than guessing at one.
+	TagOverlap float64
 }
 
 // SeedContribution is one of the top-3 contributing seeds for a candidate.
@@ -49,7 +58,10 @@ type SeedContribution struct {
 // seed path only — the likes + completed-plays fallback used while the user
 // has no taste-profile rows yet. Once the profile is populated it seeds
 // instead, carrying its own decay, so this knob stops applying.
-func SuggestArtists(ctx context.Context, pool *pgxpool.Pool, userID pgtype.UUID, halfLifeDays float64, limit int) ([]ArtistSuggestion, error) {
+func SuggestArtists(
+	ctx context.Context, pool *pgxpool.Pool, userID pgtype.UUID,
+	halfLifeDays float64, limit int, tagOverlapWeight float64,
+) ([]ArtistSuggestion, error) {
 	if limit <= 0 || limit > 50 {
 		limit = 12
 	}
@@ -117,9 +129,71 @@ func SuggestArtists(ctx context.Context, pool *pgxpool.Pool, userID pgtype.UUID,
 			Attribution: attribution,
 		})
 	}
+
+	// Taste-tag term (#2377). Applied to the whole over-fetched pool BEFORE
+	// selection, so the rotation and diversity rules in selectSuggestions
+	// operate on taste-blended scores — boosting only the twelve already
+	// chosen by similarity would leave the actual re-ranking undone.
+	//
+	// A query error here is returned, NOT degraded past. Graceful degradation
+	// is for expected absence — no taste profile yet, no cached tags for a
+	// candidate — and both of those are handled explicitly as empty inputs
+	// below. A failing query is neither: swallowing it would hide a broken DB
+	// behind a subtly worse ranking that nothing reports.
+	tasteTags, candTags, err := loadTagInputs(ctx, q, userID, out)
+	if err != nil {
+		return nil, err
+	}
+	out = applyTagOverlap(out, candTags, tasteTags, tagOverlapWeight)
+
 	return selectSuggestions(out, limit, rotationDay(time.Now())), nil
 }
 
+// tasteTagLimit caps how many of the user's taste tags participate. The
+// profile's long tail is near-zero weight and contributes nothing after
+// normalization, so this bounds the query rather than the meaning.
+const tasteTagLimit = 50
+
+// loadTagInputs fetches both sides of the overlap comparison: the user's taste
+// tags and the cached tags for exactly the candidates in this pool.
+func loadTagInputs(
+	ctx context.Context, q *dbq.Queries, userID pgtype.UUID, pool []ArtistSuggestion,
+) (TagWeights, map[string]TagWeights, error) {
+	tasteRows, err := q.ListTasteProfileTagsForUser(ctx, dbq.ListTasteProfileTagsForUserParams{
+		UserID: userID,
+		Limit:  tasteTagLimit,
+	})
+	if err != nil {
+		return nil, nil, fmt.Errorf("suggest: taste tags: %w", err)
+	}
+	// No taste tags is a cold start, not a failure — return early and skip the
+	// candidate-tag fetch entirely, since nothing could match.
+	if len(tasteRows) == 0 {
+		return nil, nil, nil
+	}
+	taste := make(TagWeights, len(tasteRows))
+	for _, r := range tasteRows {
+		taste[r.Tag] = r.Weight
+	}
+
+	mbids := make([]string, 0, len(pool))
+	for _, s := range pool {
+		mbids = append(mbids, s.MBID)
+	}
+	tagRows, err := q.ListCandidateArtistTagsForMbids(ctx, mbids)
+	if err != nil {
+		return nil, nil, fmt.Errorf("suggest: candidate tags: %w", err)
+	}
+	byCandidate := make(map[string]TagWeights, len(pool))
+	for _, r := range tagRows {
+		if byCandidate[r.CandidateMbid] == nil {
+			byCandidate[r.CandidateMbid] = TagWeights{}
+		}
+		byCandidate[r.CandidateMbid][r.Tag] = r.Weight
+	}
+	return taste, byCandidate, nil
+}
+
 // Pool multiplier: how many scored candidates to fetch per slot shown, so the
 // rotation has somewhere to rotate. 4x keeps a day's deck genuinely different
 // from yesterday's without pulling the whole long tail (whose scores are noise)