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/playlists/system.go

@@ -282,6 +282,11 @@ var systemPlaylistRegistry = []systemPlaylistKind{
 	{Key: "for_you", Singleton: true, Produce: produceForYou},
 	{Key: "songs_like_artist", Singleton: false, Produce: produceSeedMixes},
 	{Key: "discover", Singleton: true, Produce: produceDiscover},
+	{Key: "deep_cuts", Singleton: true, Produce: produceDeepCuts},
+	{Key: "rediscover", Singleton: true, Produce: produceRediscover},
+	{Key: "new_for_you", Singleton: true, Produce: produceNewForYou},
+	{Key: "on_this_day", Singleton: true, Produce: produceOnThisDay},
+	{Key: "first_listens", Singleton: true, Produce: produceFirstListens},
 }
 
 // produceForYou: today's seed from the user's top-5 played tracks