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_mixes.sql

@@ -38,24 +38,46 @@ SELECT t.id, t.album_id, t.artist_id
 
 -- name: ListRediscoverTracks :many
 -- #420 Rediscover: tracks the user played a lot (>=5 non-skip) but
--- not in the last 6 months. Ordered by historical affection.
--- $1 user_id.
+-- has drifted away from. Tiered so a young library still gets a mix:
+--   tier 0  not played in the last 6 months (true rediscovery)
+--   tier 1  (only if tier 0 empty) not played in the last 30 days
+-- Ordered by historical affection. $1 user_id.
 WITH stats AS (
-    SELECT track_id, COUNT(*) AS c, MAX(started_at) AS last_at
-      FROM play_events
-     WHERE user_id = $1 AND was_skipped = false
-     GROUP BY track_id
+    SELECT pe.track_id, COUNT(*) AS c, MAX(pe.started_at) AS last_at
+      FROM play_events pe
+     WHERE pe.user_id = $1 AND pe.was_skipped = false
+     GROUP BY pe.track_id
+),
+deep AS (
+    SELECT t.id, t.album_id, t.artist_id, s.c, 0 AS tier
+      FROM tracks t
+      JOIN stats s ON s.track_id = t.id
+     WHERE s.c >= 5
+       AND s.last_at <= now() - interval '6 months'
+       AND NOT EXISTS (
+           SELECT 1 FROM lidarr_quarantine q
+            WHERE q.user_id = $1 AND q.track_id = t.id
+       )
+),
+shallow AS (
+    SELECT t.id, t.album_id, t.artist_id, s.c, 1 AS tier
+      FROM tracks t
+      JOIN stats s ON s.track_id = t.id
+     WHERE s.c >= 5
+       AND s.last_at <= now() - interval '30 days'
+       AND NOT EXISTS (
+           SELECT 1 FROM lidarr_quarantine q
+            WHERE q.user_id = $1 AND q.track_id = t.id
+       )
 )
-SELECT t.id, t.album_id, t.artist_id
-  FROM tracks t
-  JOIN stats s ON s.track_id = t.id
- WHERE s.c >= 5
-   AND s.last_at <= now() - interval '6 months'
-   AND NOT EXISTS (
-       SELECT 1 FROM lidarr_quarantine q
-        WHERE q.user_id = $1 AND q.track_id = t.id
-   )
- ORDER BY s.c DESC, t.id
+SELECT id, album_id, artist_id
+  FROM (
+    SELECT id, album_id, artist_id, c, tier FROM deep
+    UNION ALL
+    SELECT id, album_id, artist_id, c, tier FROM shallow
+     WHERE NOT EXISTS (SELECT 1 FROM deep)
+  ) u
+ ORDER BY tier, c DESC, id
  LIMIT 200;
 
 -- name: ListNewForYouTracks :many
@@ -87,16 +109,19 @@ SELECT t.id, t.album_id, t.artist_id
 
 -- name: ListOnThisDayTracks :many
 -- #422 On This Day: tracks the user played around this calendar date
--- (±7 day-of-year) in the past, excluding the very recent (>60 days
+-- (±10 day-of-year) in the past, excluding the very recent (>30 days
 -- ago) so it's nostalgic, not just "last week". Weighted by how much
--- they were played in those windows.
+-- they were played in those windows. Floor relaxed from 60→30 days
+-- and window ±7→±10 so it surfaces on a months-old library instead
+-- of needing a full year of history; still skips cleanly (no rows →
+-- no playlist) when there's no qualifying history yet.
 -- $1 user_id, $2 date string.
 WITH windowed AS (
     SELECT track_id, COUNT(*) AS c
       FROM play_events
      WHERE user_id = $1 AND was_skipped = false
-       AND started_at < now() - interval '60 days'
-       AND ABS(EXTRACT(DOY FROM started_at) - EXTRACT(DOY FROM now())) <= 7
+       AND started_at < now() - interval '30 days'
+       AND ABS(EXTRACT(DOY FROM started_at) - EXTRACT(DOY FROM now())) <= 10
      GROUP BY track_id
 )
 SELECT t.id, t.album_id, t.artist_id