feat(playlists): #411 R3 — five discovery mixes (#419-423)

Each is one candidate query + one registry entry; zero client work
(R2 made tiles/refresh/shuffle generic, all are singleton kinds so
web's server `refreshable` flag and Flutter's derived getter both
light up automatically).

- deep_cuts (#419): <=2-play tracks from liked / heavily-played
  artists; diversity-capped.
- rediscover (#420): >=5-play tracks not heard in 6 months, by
  historical affection.
- new_for_you (#421): tracks from albums added <=30d whose artist
  the user likes/plays; album-coherent (no cap).
- on_this_day (#422): tracks played within ±7 day-of-year in prior
  windows (>60d ago), weighted by play count.
- first_listens (#423): never-played albums, tiered liked-artist →
  played-artist → rest; album-coherent.

system_mixes.go producers mirror the Discover model (SQL gives the
ranking; finishMix caps+truncates to 100 to match For-You/Discover
shuffle depth; album-coherent mixes skip the cap). Builder query
failure is non-fatal (logged, yields no playlist) like Discover.
Existing system_test existence checks are unaffected.

Closes the #411 system-playlists-v2 umbrella's new-types thread.

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

internal/db/queries/system_mixes.sql

@@ -0,0 +1,145 @@
+-- Candidate queries for the discovery system-playlist mixes
+-- (#419-423). Each returns (id, album_id, artist_id) rows; the Go
+-- producer caps + truncates and converts to rankedCandidate. Daily-
+-- deterministic md5(id||dateStr) ordering (where used) keeps a mix
+-- stable within a day while rotating across days; shuffle-on-play
+-- gives per-play variety on top.
+
+-- name: ListDeepCutsTracks :many
+-- #419 Deep Cuts: low-play tracks (<=2 plays) from artists the user
+-- has liked OR played heavily (>=5 non-skip plays across the artist).
+-- $1 user_id, $2 date string.
+WITH affinity_artists AS (
+    SELECT artist_id FROM general_likes_artists WHERE user_id = $1
+    UNION
+    SELECT t.artist_id
+      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.artist_id
+    HAVING COUNT(*) >= 5
+),
+play_counts AS (
+    SELECT track_id, COUNT(*) AS c
+      FROM play_events
+     WHERE user_id = $1 AND was_skipped = false
+     GROUP BY track_id
+)
+SELECT t.id, t.album_id, t.artist_id
+  FROM tracks t
+  JOIN affinity_artists aa ON aa.artist_id = t.artist_id
+  LEFT JOIN play_counts pc ON pc.track_id = t.id
+ WHERE COALESCE(pc.c, 0) <= 2
+   AND NOT EXISTS (
+       SELECT 1 FROM lidarr_quarantine q
+        WHERE q.user_id = $1 AND q.track_id = t.id
+   )
+ ORDER BY md5(t.id::text || $2::text)
+ LIMIT 200;
+
+-- 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.
+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 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
+ LIMIT 200;
+
+-- name: ListNewForYouTracks :many
+-- #421 New for you: tracks from albums added in the last 30 days
+-- whose artist the user has liked OR played (>=3 non-skip). Album-
+-- coherent (newest album first, then disc/track) — the producer does
+-- NOT diversity-cap these; they're meant as whole-album discovery.
+-- $1 user_id.
+WITH affinity_artists AS (
+    SELECT artist_id FROM general_likes_artists WHERE user_id = $1
+    UNION
+    SELECT t.artist_id
+      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.artist_id
+    HAVING COUNT(*) >= 3
+)
+SELECT t.id, t.album_id, t.artist_id
+  FROM tracks t
+  JOIN albums al ON al.id = t.album_id
+  JOIN affinity_artists aa ON aa.artist_id = al.artist_id
+ WHERE al.created_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
+   )
+ ORDER BY al.created_at DESC, t.disc_number NULLS FIRST, t.track_number NULLS FIRST
+ LIMIT 200;
+
+-- 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
+-- ago) so it's nostalgic, not just "last week". Weighted by how much
+-- they were played in those windows.
+-- $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
+     GROUP BY track_id
+)
+SELECT t.id, t.album_id, t.artist_id
+  FROM tracks t
+  JOIN windowed w ON w.track_id = t.id
+ WHERE NOT EXISTS (
+       SELECT 1 FROM lidarr_quarantine q
+        WHERE q.user_id = $1 AND q.track_id = t.id
+   )
+ ORDER BY w.c DESC, md5(t.id::text || $2::text)
+ LIMIT 200;
+
+-- name: ListFirstListensTracks :many
+-- #423 First Listens: albums the user has never played any track of.
+-- Tiered: liked-artist albums first, then played-artist albums, then
+-- the rest — album-coherent within each tier. Not diversity-capped
+-- (whole-album discovery). $1 user_id.
+WITH heard_albums AS (
+    SELECT DISTINCT t.album_id
+      FROM play_events pe JOIN tracks t ON t.id = pe.track_id
+     WHERE pe.user_id = $1
+),
+played_artists AS (
+    SELECT DISTINCT t.artist_id
+      FROM play_events pe JOIN tracks t ON t.id = pe.track_id
+     WHERE pe.user_id = $1
+)
+SELECT t.id, t.album_id, t.artist_id
+  FROM tracks t
+  JOIN albums al ON al.id = t.album_id
+ WHERE NOT EXISTS (SELECT 1 FROM heard_albums h WHERE h.album_id = al.id)
+   AND NOT EXISTS (
+       SELECT 1 FROM lidarr_quarantine q
+        WHERE q.user_id = $1 AND q.track_id = t.id
+   )
+ ORDER BY
+   (CASE
+      WHEN EXISTS (
+        SELECT 1 FROM general_likes_artists gla
+         WHERE gla.user_id = $1 AND gla.artist_id = al.artist_id
+      ) THEN 0
+      WHEN al.artist_id IN (SELECT artist_id FROM played_artists) THEN 1
+      ELSE 2
+    END),
+   al.id, t.disc_number NULLS FIRST, t.track_number NULLS FIRST
+ LIMIT 300;