fix(playlists): robust For-You seed + deep fill; young-library mix fallbacks

For-You silently vanished after ~7 days of not listening (seed query
required a non-skip play in the last 7 days) and capped at ~40 on
self-hosted libraries with no ListenBrainz similarity data.

- PickTopPlayedTracksForUser: tiered seed — last 30d top plays, else
  all-time top plays, else liked tracks. For-You only disappears now
  if the account has zero plays AND zero likes.
- produceForYou uses deeper candidate source limits (raised random/
  tag/similar K) so it reaches ~100 even with empty lb_similar /
  similar_artists; richer when LB enrichment is present.
- Rediscover: tiered — 6-month-dormant, else ≥5-play 30-day-dormant.
- On This Day: floor 60→30 days, window ±7→±10 doy; still skips
  cleanly (no rows → no playlist) on insufficient history.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/playlists/system.go

@@ -289,6 +289,23 @@ var systemPlaylistRegistry = []systemPlaylistKind{
 	{Key: "first_listens", Singleton: true, Produce: produceFirstListens},
 }
 
+// systemForYouSourceLimits is a deeper candidate pool than the radio
+// default. On a self-hosted library without ListenBrainz similarity
+// data the lb_similar / similar_artists sources contribute nothing,
+// so the default (~130 raw, ~40 after dedup + diversity caps) can
+// never fill For-You's 100-track head/tail. The raised random/tag
+// fill keeps For-You ~100 deep regardless of LB enrichment; when LB
+// data IS present the larger lb/similar K just makes it richer.
+func systemForYouSourceLimits() recommendation.CandidateSourceLimits {
+	return recommendation.CandidateSourceLimits{
+		LBSimilar:     80,
+		SimilarArtist: 80,
+		TagOverlap:    60,
+		LikesOverlap:  40,
+		RandomFill:    150,
+	}
+}
+
 // produceForYou: today's seed from the user's top-5 played tracks
 // (rotates daily via userIDHash), similarity candidate pool, head+
 // tail composition. The base seed query failing is fatal; a
@@ -311,7 +328,7 @@ func produceForYou(
 		1, // recentlyPlayedHours — small to avoid filtering the seed's recent neighbourhood
 		zeroVec,
 		[]pgtype.UUID{forYouSeed},
-		recommendation.DefaultCandidateSourceLimits(),
+		systemForYouSourceLimits(),
 	)
 	if cerr != nil {
 		logger.Warn("system playlist: for-you candidates load failed; skipping",