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/api/playlists.go

@@ -32,7 +32,12 @@ type playlistRowView struct {
 	IsPublic      bool    `json:"is_public"`
 	Kind          string  `json:"kind"`
 	SystemVariant *string `json:"system_variant,omitempty"`
-	SeedArtistID  *string `json:"seed_artist_id,omitempty"`
+	// Refreshable: a singleton system kind addressable by the generic
+	// /system/{kind}/{refresh,shuffle} endpoints. Lets clients show
+	// the per-tile refresh affordance generically without hardcoding
+	// which kinds support it (false for user + songs_like_artist).
+	Refreshable  bool    `json:"refreshable"`
+	SeedArtistID *string `json:"seed_artist_id,omitempty"`
 	CoverURL      string  `json:"cover_url,omitempty"`
 	TrackCount    int32   `json:"track_count"`
 	DurationSec   int32   `json:"duration_sec"`
@@ -435,6 +440,7 @@ func playlistRowToView(r *playlists.PlaylistRow) playlistRowView {
 		IsPublic:      r.IsPublic,
 		Kind:          r.Kind,
 		SystemVariant: r.SystemVariant,
+		Refreshable:   r.SystemVariant != nil && playlists.RefreshableSystemKind(*r.SystemVariant),
 		TrackCount:    r.TrackCount,
 		DurationSec:   r.DurationSec,
 		CreatedAt:     formatTimestamp(r.CreatedAt),