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/db/queries/system_playlists.sql

@@ -73,20 +73,50 @@ SELECT p.artist_id,
  LIMIT 5;
 
 -- name: PickTopPlayedTracksForUser :many
--- For-You candidate seeds. Returns the user's top-5 most-played
--- non-skipped tracks in the last 7 days; tie-break by track_id for
--- determinism. The Go-side picker (pickForYouSeedForDay) chooses one
--- of the returned rows as today's seed via userIDHash so the
--- candidate pool rotates day-to-day while staying stable within a
--- day.
-SELECT t.id
-  FROM play_events pe
-  JOIN tracks t ON t.id = pe.track_id
- WHERE pe.user_id = $1
-   AND pe.started_at > now() - INTERVAL '7 days'
-   AND pe.was_skipped = false
- GROUP BY t.id
- ORDER BY COUNT(*) DESC, t.id
+-- For-You candidate seeds, tiered so For-You never silently vanishes:
+--   tier 0  top non-skip plays in the last 30 days
+--   tier 1  (only if tier 0 empty) all-time top non-skip plays
+--   tier 2  (only if tiers 0+1 empty) liked tracks
+-- Returns up to 5 ids; tie-break by track_id for determinism. The
+-- Go-side picker (pickForYouSeedForDay) rotates one per day via
+-- userIDHash. Widened from a hard 7-day window, which made For-You
+-- disappear after a week of not listening and never recover on a
+-- self-hosted library with sparse history.
+WITH recent AS (
+    SELECT t.id, COUNT(*) AS c, 0 AS tier
+      FROM play_events pe
+      JOIN tracks t ON t.id = pe.track_id
+     WHERE pe.user_id = $1
+       AND pe.started_at > now() - INTERVAL '30 days'
+       AND pe.was_skipped = false
+     GROUP BY t.id
+),
+alltime AS (
+    SELECT t.id, COUNT(*) AS c, 1 AS tier
+      FROM play_events pe
+      JOIN tracks t ON t.id = pe.track_id
+     WHERE pe.user_id = $1
+       AND pe.was_skipped = false
+     GROUP BY t.id
+),
+liked AS (
+    SELECT gl.track_id AS id, 0::bigint AS c, 2 AS tier
+      FROM general_likes gl
+     WHERE gl.user_id = $1
+),
+chosen AS (
+    SELECT id, c, tier FROM recent
+    UNION ALL
+    SELECT id, c, tier FROM alltime
+     WHERE NOT EXISTS (SELECT 1 FROM recent)
+    UNION ALL
+    SELECT id, c, tier FROM liked
+     WHERE NOT EXISTS (SELECT 1 FROM recent)
+       AND NOT EXISTS (SELECT 1 FROM alltime)
+)
+SELECT id
+  FROM chosen
+ ORDER BY tier, c DESC, id
  LIMIT 5;
 
 -- name: PickTopPlayedTrackForArtistByUser :one