refactor(playlists): #411 R2 — generic registry-driven system endpoints

Replaces the per-kind refresh/shuffle handlers with one generic
pair driven off the kind registry, in lockstep across both clients.

Server:
- systemPlaylistKind gains Singleton; RefreshableSystemKind(key)
  exported. for_you/discover singleton; songs_like_artist not.
- New generic POST /api/playlists/system/{kind}/refresh and
  GET /api/playlists/system/{kind}/shuffle ({kind} = raw
  system_variant). Non-singleton/unknown kind → 404. Deleted
  playlists_{foryou,discover}_refresh.go and the per-kind shuffle
  wrappers; serveSystemPlaylistShuffle core kept.
- playlistRowView.refreshable: server-derived flag so clients show
  the refresh affordance generically without hardcoding kinds.

Web (not drift-cached → uses the server flag):
- refreshSystem(variant) replaces refreshForYou/refreshDiscover;
  systemShuffle drops the for_you→for-you mapping (raw variant).
- PlaylistCard + detail page gate the kebab/Refresh button on
  playlist.refreshable; label is "Refresh {name}". Tests reworked;
  obsolete refresh-foryou/discover api tests deleted.

Flutter (list tiles are drift-cache-sourced → derive, no migration):
- Playlist.refreshable getter = isSystem && variant !=
  songs_like_artist (holds for all current + planned kinds).
- refreshSystem/systemShuffle use the raw variant; PlaylistCard +
  detail screen gate kebab/Regenerate/source-tagging on refreshable
  so songs_like_artist plays via get() (no by-kind endpoint).

Pure-plumbing refactor; CI verifies parity. Next (R3): the five
discovery mixes — each a candidate query + one registry entry.

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

internal/playlists/system.go

@@ -246,8 +246,29 @@ type systemPlaylistProducer func(
 ) ([]builtPlaylist, error)
 
 type systemPlaylistKind struct {
-	Key     string
-	Produce systemPlaylistProducer
+	Key string
+	// Singleton kinds produce exactly one playlist per user (For-You,
+	// Discover, the discovery mixes) and so can be addressed by kind:
+	// the generic /system/{kind}/{refresh,shuffle} endpoints + the
+	// per-tile refresh affordance only apply to these. Non-singleton
+	// kinds (songs_like_artist → up to 3 per user) are played by
+	// playlist id and have no by-kind endpoint.
+	Singleton bool
+	Produce   systemPlaylistProducer
+}
+
+// RefreshableSystemKind reports whether `key` is a known singleton
+// system-playlist kind — i.e. addressable by the generic by-kind
+// refresh/shuffle endpoints and eligible for the per-tile refresh
+// affordance. The API layer + clients use this so neither hardcodes
+// the kind list.
+func RefreshableSystemKind(key string) bool {
+	for _, k := range systemPlaylistRegistry {
+		if k.Key == key {
+			return k.Singleton
+		}
+	}
+	return false
 }
 
 // systemPlaylistRegistry is the single source of truth for which
@@ -258,9 +279,9 @@ type systemPlaylistKind struct {
 // it has no functional effect (atomic replace + per-playlist
 // collage are order-independent).
 var systemPlaylistRegistry = []systemPlaylistKind{
-	{Key: "for_you", Produce: produceForYou},
-	{Key: "songs_like_artist", Produce: produceSeedMixes},
-	{Key: "discover", Produce: produceDiscover},
+	{Key: "for_you", Singleton: true, Produce: produceForYou},
+	{Key: "songs_like_artist", Singleton: false, Produce: produceSeedMixes},
+	{Key: "discover", Singleton: true, Produce: produceDiscover},
 }
 
 // produceForYou: today's seed from the user's top-5 played tracks