feat(discover): rotate the suggestion deck daily + cap one seed's share — #2373

Second half of the reported symptom: suggestions "show the same artists until
you request one". The ranking was `ORDER BY total_score DESC` with no
randomization and no seen-state, so the only things that could ever change the
deck were a candidate entering the library or the user filing a request. The
tail of the ranking was unreachable — requesting was literally the only lever.

No SQL change was needed. The query already takes a limit, so it over-fetches a
pool (4x the slots, capped at 60) and the selection moves to Go, where it is a
pure function of (pool, limit, day) — no DB, no clock — and therefore unit
testable in the fast lane instead of behind the integration gate.

Three rules. The best few by score always lead, so the strongest matches never
rotate out of sight (For You's head/tail shape). The remaining slots are drawn
by md5(mbid + day), the same daily-stable idiom the Home rows already use:
stable within a day so pull-to-refresh doesn't reshuffle, different tomorrow,
and no stored state. And a per-seed cap keeps roughly a quarter of the deck
attributable to any one seed artist, so twelve neighbours of a single artist
can't be the whole surface.

The cap is a preference, not a quota. A user whose pool hangs off one or two
seeds would otherwise get a three-card surface — worse than the monoculture
being avoided, and exactly the vanish-or-nothing shape rule #131 exists to
prevent — so a short deck tops up in score order from what the cap set aside.
This is also what keeps the existing Top12Cap integration test honest: its
30 candidates share one seed, and without the top-up it would return 3.

Eight unit tests, including one that had to be rewritten mid-change: the first
version asserted the cap against an evenly-spread pool, where the top-N is
already diverse and the assertion could not fail. It now uses a skewed pool
where one seed owns the entire top of the ranking, which is the only shape that
actually exercises a cap.

Dropped two //nolint:gosec directives added in passing — gosec isn't in
.golangci.yml, so they suppressed nothing and only implied a check that runs.

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

internal/recommendation/suggestions_select_test.go

@@ -0,0 +1,170 @@
+package recommendation
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/jackc/pgx/v5/pgtype"
+)
+
+// selectSuggestions is deliberately pure (no DB, no clock), so the rotation and
+// diversity rules from slice #2373 are covered here in the fast lane rather
+// than behind the integration gate.
+
+func seedID(n byte) pgtype.UUID {
+	var u pgtype.UUID
+	u.Bytes[0] = n
+	u.Valid = true
+	return u
+}
+
+// candidate builds a pool entry attributed to the given dominant seed.
+func candidate(mbid string, score float64, dominant byte) ArtistSuggestion {
+	return ArtistSuggestion{
+		MBID:  mbid,
+		Name:  mbid,
+		Score: score,
+		Attribution: []SeedContribution{
+			{ArtistID: seedID(dominant), Contribution: score},
+		},
+	}
+}
+
+// poolOf returns n candidates in descending score order, spread across
+// `seeds` distinct dominant seeds.
+func poolOf(n int, seeds int) []ArtistSuggestion {
+	out := make([]ArtistSuggestion, 0, n)
+	for i := 0; i < n; i++ {
+		out = append(out, candidate(fmt.Sprintf("mbid-%02d", i), 1.0-float64(i)*0.01, byte(i%seeds)))
+	}
+	return out
+}
+
+func mbids(in []ArtistSuggestion) []string {
+	out := make([]string, 0, len(in))
+	for _, s := range in {
+		out = append(out, s.MBID)
+	}
+	return out
+}
+
+func TestSelectSuggestions_ReturnsPoolUnchangedWhenNotOverfetched(t *testing.T) {
+	pool := poolOf(5, 5)
+	got := selectSuggestions(pool, 12, "2026-08-01")
+	if len(got) != 5 {
+		t.Fatalf("len = %d, want 5 (nothing to rotate)", len(got))
+	}
+	if got[0].MBID != "mbid-00" {
+		t.Errorf("first = %q, want mbid-00", got[0].MBID)
+	}
+}
+
+func TestSelectSuggestions_FillsTheRequestedLimit(t *testing.T) {
+	got := selectSuggestions(poolOf(48, 8), 12, "2026-08-01")
+	if len(got) != 12 {
+		t.Fatalf("len = %d, want 12", len(got))
+	}
+}
+
+// The reported symptom: the deck must change day to day without the user
+// requesting anything.
+func TestSelectSuggestions_RotatesAcrossDays(t *testing.T) {
+	pool := poolOf(48, 8)
+	day1 := mbids(selectSuggestions(pool, 12, "2026-08-01"))
+	day2 := mbids(selectSuggestions(pool, 12, "2026-08-02"))
+
+	if fmt.Sprint(day1) == fmt.Sprint(day2) {
+		t.Fatalf("deck identical across days: %v", day1)
+	}
+	// ...but stable WITHIN a day, or the surface would reshuffle on every
+	// pull-to-refresh, which reads as broken rather than fresh.
+	again := mbids(selectSuggestions(pool, 12, "2026-08-01"))
+	if fmt.Sprint(day1) != fmt.Sprint(again) {
+		t.Errorf("same day differed:\n %v\n %v", day1, again)
+	}
+}
+
+// The strongest matches should never rotate out of sight.
+func TestSelectSuggestions_HeadIsStableTopScorers(t *testing.T) {
+	pool := poolOf(48, 8)
+	for _, day := range []string{"2026-08-01", "2026-08-02", "2026-09-15"} {
+		got := selectSuggestions(pool, 12, day)
+		if got[0].MBID != "mbid-00" {
+			t.Errorf("day %s: first = %q, want mbid-00 (top score leads)", day, got[0].MBID)
+		}
+	}
+}
+
+// #2367 mechanism 4: all twelve slots could be neighbours of one artist.
+//
+// The pool is deliberately SKEWED — one seed owns the entire top of the score
+// ranking — because that is the only shape where a cap can bite. With scores
+// spread evenly across seeds the top-N is already diverse and the cap is
+// untestable (an earlier version of this test asserted against an even pool and
+// could not fail).
+func TestSelectSuggestions_CapsOneDominantSeed(t *testing.T) {
+	const dominantRun = 20
+	pool := make([]ArtistSuggestion, 0, 48)
+	for i := 0; i < dominantRun; i++ { // seed 0 owns the 20 best scores
+		pool = append(pool, candidate(fmt.Sprintf("dom-%02d", i), 1.0-float64(i)*0.01, 0))
+	}
+	for i := 0; i < 28; i++ { // seeds 1..9 hold everything below
+		pool = append(pool, candidate(fmt.Sprintf("oth-%02d", i), 0.80-float64(i)*0.01, byte(1+i%9)))
+	}
+
+	got := selectSuggestions(pool, 12, "2026-08-01")
+	if len(got) != 12 {
+		t.Fatalf("len = %d, want 12", len(got))
+	}
+
+	perSeed := map[pgtype.UUID]int{}
+	for _, s := range got {
+		perSeed[s.Attribution[0].ArtistID]++
+	}
+	// Uncapped, the top 12 by score would be 12 of seed 0's neighbours.
+	if n := perSeed[seedID(0)]; n > maxPerSeedFor(12) {
+		t.Errorf("dominant seed owns %d of 12 slots, want <= %d: %v",
+			n, maxPerSeedFor(12), mbids(got))
+	}
+	if len(perSeed) < 4 {
+		t.Errorf("deck spans only %d seeds, want >= 4: %v", len(perSeed), mbids(got))
+	}
+}
+
+// Diversity must not starve the deck (rule #131): a pool hanging off a single
+// seed should still fill, not collapse to maxPerSeed entries.
+func TestSelectSuggestions_SingleSeedPoolStillFills(t *testing.T) {
+	got := selectSuggestions(poolOf(30, 1), 12, "2026-08-01")
+	if len(got) != 12 {
+		t.Fatalf("len = %d, want 12 (cap must not starve the deck)", len(got))
+	}
+	if got[0].MBID != "mbid-00" {
+		t.Errorf("first = %q, want mbid-00", got[0].MBID)
+	}
+}
+
+// A candidate with no attribution has no seed to blame, so it must never be
+// held back by the diversity cap.
+func TestPartitionPerSeed_UnattributedNeverCapped(t *testing.T) {
+	pool := []ArtistSuggestion{
+		candidate("a", 0.9, 1),
+		candidate("b", 0.8, 1),
+		{MBID: "orphan", Score: 0.1},
+	}
+	preferred, overflow := partitionPerSeed(pool, 1)
+	if len(preferred) != 2 || preferred[1].MBID != "orphan" {
+		t.Errorf("preferred = %v, want [a orphan]", mbids(preferred))
+	}
+	if len(overflow) != 1 || overflow[0].MBID != "b" {
+		t.Errorf("overflow = %v, want [b]", mbids(overflow))
+	}
+}
+
+func TestPoolSizeFor_OverfetchesButIsBounded(t *testing.T) {
+	if got := poolSizeFor(12); got != 48 {
+		t.Errorf("poolSizeFor(12) = %d, want 48", got)
+	}
+	if got := poolSizeFor(50); got != suggestionPoolMax {
+		t.Errorf("poolSizeFor(50) = %d, want the %d ceiling", got, suggestionPoolMax)
+	}
+}