fix(server): daily-rotate all deterministic mixes + diversity top-up fallback

Operator feedback on the prior unification commit (7473e98d):

1. NewForYou should daily-rotate alongside Rediscover and FirstListens.
   The 'newest album first regardless of day' intent was the wrong
   call - operator wants visible day-over-day movement on every
   deterministic mix surface. Spec flipped to dailyRotate: true.

2. Diversity caps (<=2 per album / <=3 per artist) on every mix, not
   just the historically-diverse ones. The 2-per-album limit has
   helped a lot on the operator's library; extending it to NewForYou
   and FirstListens (previously album-coherent / no cap) surfaces
   more distinct albums per day. Spec flipped to diversify: true on
   all five.

3. Fallback when diversity caps strip the pool below the 100-track
   target: finishMix now calls topUpFromRaw, which appends non-capped
   tracks from the raw SQL pool (preserving original ranked order +
   skipping duplicates) until the target is hit or the pool runs out.
   On rich libraries the cap yields >= 100 and top-up never runs; on
   thin / album-heavy libraries we ship a partly-diversified 100
   instead of a strictly-diversified 40.

Net effect: every deterministic mix now rotates day-over-day, every
mix gets the same diversity treatment (with graceful degradation),
and the producer surface stays a single factory over a spec list.

internal/playlists/system_mixes.go

@@ -13,8 +13,8 @@ import (
 
 // Discovery mixes (#419-423). One generic producer + per-mix spec.
 // Replaces the five near-identical produceXxx functions that all
-// fetched ranked (id, album_id, artist_id) rows, optionally
-// diversified, truncated, and emitted a single playlist.
+// fetched ranked (id, album_id, artist_id) rows, diversified,
+// truncated, and emitted a single playlist.
 //
 // Day-keying is a per-mix property captured by `dailyRotate`:
 //
@@ -22,18 +22,19 @@ import (
 //     ORDER BY md5(t.id::text || $2::text), so the Go producer keeps
 //     SQL order. `dailyRotate: false`.
 //
-//   - Rediscover / FirstListens — SQL accepts only $1 user_id and
-//     produces deterministic ordering. Same content day-over-day
-//     until library state shifts. `dailyRotate: true` applies a
-//     daily-deterministic rotate-left of the pool BEFORE diversify+
-//     truncate so each day's top-100 surfaces a different slice
-//     while contiguous-block ordering within each slice is preserved
-//     (matters for FirstListens which is album-coherent).
+//   - Rediscover / NewForYou / FirstListens — SQL accepts only $1
+//     user_id and produces deterministic ordering. `dailyRotate:
+//     true` applies a daily-deterministic rotate-left of the pool
+//     BEFORE diversify+truncate so each day's top-100 surfaces a
+//     different slice while contiguous-block ordering within each
+//     slice is preserved (matters for FirstListens / NewForYou which
+//     are album-coherent — rotation walks the album boundary cleanly
+//     rather than scrambling within an album).
 //
-//   - NewForYou — SQL produces a newest-album-first ordering whose
-//     intent is "see what's new". Day-over-day same content is
-//     correct UX: the user's "what's new" list shouldn't rotate. The
-//     spec carries `dailyRotate: false` deliberately.
+// Diversity is `true` for every mix: per-album <= 2 / per-artist <= 3.
+// On thin libraries where the cap would chop the pool below 100,
+// finishMix tops up from the uncapped raw pool so the mix still
+// ships a full-length playlist — see topUpFromRaw.
 
 // discoveryMixLen caps each mix at the same depth as For-You /
 // Discover so shuffle-on-play has a varied pool within a day.
@@ -139,7 +140,7 @@ var discoveryMixSpecs = []discoveryMixSpec{
 	},
 	{
 		name: "New for you", variant: "new_for_you",
-		diversify: false, dailyRotate: false, // newest-first is the intent
+		diversify: true, dailyRotate: true, // operator wants daily rotation on all deterministic mixes
 		fetch: func(ctx context.Context, q *dbq.Queries, uid pgtype.UUID, _ string) ([]discoverTrack, error) {
 			rows, err := q.ListNewForYouTracks(ctx, uid)
 			if err != nil {
@@ -171,7 +172,7 @@ var discoveryMixSpecs = []discoveryMixSpec{
 	},
 	{
 		name: "First listens", variant: "first_listens",
-		diversify: false, dailyRotate: true, // SQL has no date arg; album-coherent so rotate (not shuffle)
+		diversify: true, dailyRotate: true, // SQL has no date arg; daily rotate + diversity top-up
 		fetch: func(ctx context.Context, q *dbq.Queries, uid pgtype.UUID, _ string) ([]discoverTrack, error) {
 			rows, err := q.ListFirstListensTracks(ctx, uid)
 			if err != nil {
@@ -186,17 +187,30 @@ var discoveryMixSpecs = []discoveryMixSpec{
 	},
 }
 
-// finishMix caps (per-album<=2 / per-artist<=3) when diversify is
-// set, truncates to discoveryMixLen, and converts to the insert
-// type. Album-coherent mixes (New for you, First Listens) pass
-// diversify=false so whole albums survive.
+// finishMix applies diversity caps (per-album <= 2 / per-artist <= 3)
+// when diversify is set, with a top-up fallback when caps strip the
+// pool below discoveryMixLen: the capped result is filled out with
+// non-capped tracks (preserving original SQL order) until the target
+// is hit or the raw pool runs out.
+//
+// The fallback matters on small / album-heavy libraries — the cap
+// can chop a 200-row pool down to 40, and we'd rather ship a partly-
+// diversified 100 than a strictly-diversified 40. On rich libraries
+// the cap yields >= 100 and the top-up path never runs.
 func finishMix(rows []discoverTrack, diversify bool) []rankedCandidate {
-	pool := rows
+	var pool []discoverTrack
 	if diversify {
-		pool = capByAlbumAndArtist(pool)
-	}
-	if len(pool) > discoveryMixLen {
-		pool = pool[:discoveryMixLen]
+		capped := capByAlbumAndArtist(rows)
+		if len(capped) >= discoveryMixLen {
+			pool = capped[:discoveryMixLen]
+		} else {
+			pool = topUpFromRaw(capped, rows, discoveryMixLen)
+		}
+	} else {
+		pool = rows
+		if len(pool) > discoveryMixLen {
+			pool = pool[:discoveryMixLen]
+		}
 	}
 	if len(pool) == 0 {
 		return nil
@@ -208,6 +222,32 @@ func finishMix(rows []discoverTrack, diversify bool) []rankedCandidate {
 	return tracks
 }
 
+// topUpFromRaw appends non-capped tracks from raw (in their original
+// order) onto capped, skipping any already present, until the result
+// reaches target or raw is exhausted. Preserves SQL ranking semantics
+// for the non-diverse fill so the topped-up tail still trends best-
+// first within each album.
+func topUpFromRaw(capped, raw []discoverTrack, target int) []discoverTrack {
+	if len(capped) >= target {
+		return capped[:target]
+	}
+	seen := make(map[pgtype.UUID]struct{}, len(capped))
+	for _, t := range capped {
+		seen[t.ID] = struct{}{}
+	}
+	pool := capped
+	for _, t := range raw {
+		if _, in := seen[t.ID]; in {
+			continue
+		}
+		pool = append(pool, t)
+		if len(pool) >= target {
+			break
+		}
+	}
+	return pool
+}
+
 // emit wraps the finished track list in a single builtPlaylist (the
 // discovery mixes are all singletons). nil tracks → no playlist.
 func emit(name, variant string, tracks []rankedCandidate) []builtPlaylist {