fix(recommendation): Rediscover no longer ships a one-song playlist (#1246)

Confirmed against prod: exactly one track (17 plays, cold since May 21)
met the c>=5 + 30d-cold bar, and three process defects turned that into
a 1-track playlist instead of the locked placeholder.

- ListRediscoverTracks: collapse the two-tier UNION into one blended
  pool. The old shallow-tier gate (WHERE NOT EXISTS deep) was
  all-or-nothing — one 6-month row suppressed the entire 30-day tier —
  and deep was a strict subset of shallow anyway. Eligibility drops to
  >=3 non-skip plays (on a weeks-old history the >=5-play tracks are
  precisely the ones still in rotation); ordering prefers >=6mo cold,
  then >=5 plays, then raw count.
- Minimum viable mix floor for all five discovery mixes: below
  minLen (15; 5 for the album-coherent NewForYou/FirstListens) the
  variant is withheld so Home renders the 'listen more to unlock'
  placeholder instead of a mix that reads as built-wrong.
- /api/events: clamp client-supplied 'at' to [user.created_at,
  now+5m]. Unbounded client clocks could write arbitrarily old plays
  and poison the 6-month ordering (prod data verified clean — no
  scrub needed).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TsF3cNoKrqCYsU78cXC8U6

internal/playlists/system_mixes.go

@@ -40,6 +40,17 @@ import (
 // Discover so shuffle-on-play has a varied pool within a day.
 const discoveryMixLen = 100
 
+// Minimum viable mix sizes (issue #1246): below the floor the variant
+// is withheld entirely so Home renders its "listen more to unlock"
+// placeholder — a playlist with a couple of songs reads as a build
+// bug, not a mix. Album-coherent mixes (NewForYou / FirstListens) get
+// a lower floor because one legitimate new album (~5+ tracks) is a
+// useful mix on its own; the scattered mixes need more to feel real.
+const (
+	discoveryMixMinLen      = 15
+	discoveryMixMinLenAlbum = 5
+)
+
 // discoveryMixSpec describes one discovery mix. The unified producer
 // reads the spec and runs a single code path for all variants.
 type discoveryMixSpec struct {
@@ -55,6 +66,11 @@ type discoveryMixSpec struct {
 	// or when day-over-day stability is the intended UX (NewForYou).
 	dailyRotate bool
 
+	// minLen is the minimum viable mix size: a finished pool below it
+	// is withheld (no playlist row) so the client renders the locked
+	// placeholder instead of a mix that looks built-wrong (#1246).
+	minLen int
+
 	// fetch returns the raw ranked rows. dateStr is supplied for
 	// queries that accept it (passed as the second positional arg
 	// historically); queries that don't accept it ignore the param.
@@ -79,7 +95,13 @@ func produceDiscoveryMix(spec discoveryMixSpec) systemPlaylistProducer {
 		if spec.dailyRotate {
 			pool = rotateForDay(pool, userID, dateStr)
 		}
-		return emit(spec.name, spec.variant, finishMix(pool, spec.diversify)), nil
+		tracks := finishMix(pool, spec.diversify)
+		if len(tracks) < spec.minLen {
+			logger.Info("system playlist: "+spec.variant+" below minimum viable size; withholding",
+				"user_id", uuidStringPL(userID), "pool", len(tracks), "min", spec.minLen)
+			return nil, nil
+		}
+		return emit(spec.name, spec.variant, tracks), nil
 	}
 }
 
@@ -109,6 +131,7 @@ var discoveryMixSpecs = []discoveryMixSpec{
 	{
 		name: "Deep Cuts", variant: "deep_cuts",
 		diversify: true, dailyRotate: false, // SQL day-keys via md5(id||$2)
+		minLen: discoveryMixMinLen,
 		fetch: func(ctx context.Context, q *dbq.Queries, uid pgtype.UUID, ds string) ([]discoverTrack, error) {
 			rows, err := q.ListDeepCutsTracks(ctx, dbq.ListDeepCutsTracksParams{
 				UserID: uid, Column2: ds,
@@ -126,6 +149,7 @@ var discoveryMixSpecs = []discoveryMixSpec{
 	{
 		name: "Rediscover", variant: "rediscover",
 		diversify: true, dailyRotate: true, // SQL has no date arg
+		minLen: discoveryMixMinLen,
 		fetch: func(ctx context.Context, q *dbq.Queries, uid pgtype.UUID, _ string) ([]discoverTrack, error) {
 			rows, err := q.ListRediscoverTracks(ctx, uid)
 			if err != nil {
@@ -141,6 +165,7 @@ var discoveryMixSpecs = []discoveryMixSpec{
 	{
 		name: "New for you", variant: "new_for_you",
 		diversify: true, dailyRotate: true, // operator wants daily rotation on all deterministic mixes
+		minLen: discoveryMixMinLenAlbum,
 		fetch: func(ctx context.Context, q *dbq.Queries, uid pgtype.UUID, _ string) ([]discoverTrack, error) {
 			rows, err := q.ListNewForYouTracks(ctx, uid)
 			if err != nil {
@@ -156,6 +181,7 @@ var discoveryMixSpecs = []discoveryMixSpec{
 	{
 		name: "On this day", variant: "on_this_day",
 		diversify: true, dailyRotate: false, // SQL day-keys via md5(id||$2)
+		minLen: discoveryMixMinLen,
 		fetch: func(ctx context.Context, q *dbq.Queries, uid pgtype.UUID, ds string) ([]discoverTrack, error) {
 			rows, err := q.ListOnThisDayTracks(ctx, dbq.ListOnThisDayTracksParams{
 				UserID: uid, Column2: ds,
@@ -173,6 +199,7 @@ var discoveryMixSpecs = []discoveryMixSpec{
 	{
 		name: "First listens", variant: "first_listens",
 		diversify: true, dailyRotate: true, // SQL has no date arg; daily rotate + diversity top-up
+		minLen: discoveryMixMinLenAlbum,
 		fetch: func(ctx context.Context, q *dbq.Queries, uid pgtype.UUID, _ string) ([]discoverTrack, error) {
 			rows, err := q.ListFirstListensTracks(ctx, uid)
 			if err != nil {