feat(android): PlaylistDetail Regenerate button + refreshSystem endpoint

Audit v2 #15. System-playlist detail screens now expose a
"Regenerate" OutlinedButton next to Play + Shuffle when the playlist
is refreshable (every system variant except songs_like_artist —
mirrors Flutter's PlaylistRef.refreshable).

Plumbing:
* PlaylistsApi.refreshSystem(variant) → RefreshSystemResponse with
  optional playlist_id (server rotates the uuid; null when library
  can't seed a build).
* PlaylistsRepository.refreshSystemPlaylist returns the new id and
  invalidates the list cache so home-row tiles point at the new uuid.
* PlaylistDetailViewModel.regenerate calls into the repo and emits
  the new id on a Channel.
* Screen collects the channel and navController.navigate-with-popUpTo
  replaces the current detail route, so the back stack doesn't hold
  the now-404'd old uuid.

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

android/app/src/main/java/com/fabledsword/minstrel/api/endpoints/PlaylistsApi.kt

@@ -38,6 +38,16 @@ interface PlaylistsApi {
         @Path("id") playlistId: String,
         @Body body: AppendTracksRequest,
     )
+
+    /**
+     * POST /api/playlists/system/{kind}/refresh — rebuild the caller's
+     * system playlist of the given variant ("for_you" / "discover" /
+     * etc.). Returns the new playlist UUID (the rebuild rotates the
+     * id); playlistId is null when the library is empty / build can't
+     * pick seed tracks. The kind is the raw system_variant.
+     */
+    @POST("api/playlists/system/{kind}/refresh")
+    suspend fun refreshSystem(@Path("kind") variant: String): RefreshSystemResponse
 }
 
 /**
@@ -48,3 +58,13 @@ interface PlaylistsApi {
 data class AppendTracksRequest(
     @SerialName("track_ids") val trackIds: List<String>,
 )
+
+/**
+ * Response body for the system-refresh endpoint. `playlistId` is the
+ * uuid of the rebuilt playlist, or null when the build had nothing
+ * to seed from.
+ */
+@Serializable
+data class RefreshSystemResponse(
+    @SerialName("playlist_id") val playlistId: String? = null,
+)

android/app/src/main/java/com/fabledsword/minstrel/models/Playlist.kt

@@ -22,6 +22,14 @@ data class PlaylistRef(
     val ownerUsername: String = "",
 ) {
     val isSystem: Boolean get() = systemVariant != null
+
+    /**
+     * Whether the "Regenerate" affordance applies. Mirrors Flutter's
+     * `Playlist.refreshable`: every system playlist EXCEPT
+     * songs_like_artist (server's *songs-like-X* family doesn't
+     * expose a refresh trigger; rebuilds are scheduler-driven).
+     */
+    val refreshable: Boolean get() = isSystem && systemVariant != "songs_like_artist"
 }
 
 /**

android/app/src/main/java/com/fabledsword/minstrel/playlists/data/PlaylistsRepository.kt

@@ -112,6 +112,19 @@ class PlaylistsRepository @Inject constructor(
      * is already in the playlist, the IGNORE on conflict leaves the
      * existing position unchanged.
      */
+    /**
+     * Rebuilds the caller's system playlist of [variant]. The server
+     * rotates the playlist's UUID on rebuild — the returned id (null
+     * when the library can't seed a build) is what callers should
+     * navigate to. List cache is refreshed so home-row tiles point at
+     * the fresh uuid.
+     */
+    suspend fun refreshSystemPlaylist(variant: String): String? {
+        val response = api.refreshSystem(variant)
+        runCatching { refreshList() }
+        return response.playlistId
+    }
+
     suspend fun appendTrack(playlistId: String, trackId: String): AppendOutcome {
         val nextPos = (playlistTrackDao.maxPosition(playlistId) ?: -1) + 1
         playlistTrackDao.insertOrIgnore(

android/app/src/main/java/com/fabledsword/minstrel/playlists/ui/PlaylistDetailScreen.kt

@@ -54,6 +54,7 @@ import com.composables.icons.lucide.ArrowLeft
 import com.composables.icons.lucide.ListMusic
 import com.composables.icons.lucide.Lucide
 import com.composables.icons.lucide.Play
+import com.composables.icons.lucide.RefreshCw
 import com.composables.icons.lucide.Shuffle
 import com.fabledsword.minstrel.events.EventsStream
 import com.fabledsword.minstrel.events.LiveEvent
@@ -125,6 +126,14 @@ class PlaylistDetailViewModel @Inject constructor(
     private val deletedChannel = Channel<Unit>(Channel.BUFFERED)
     val deleted: Flow<Unit> = deletedChannel.receiveAsFlow()
 
+    /**
+     * One-shot signal: a system playlist regenerate landed and the
+     * server rotated the uuid. Screen replaces the current detail
+     * route with the new id so the user sees the fresh list.
+     */
+    private val regeneratedChannel = Channel<String>(Channel.BUFFERED)
+    val regenerated: Flow<String> = regeneratedChannel.receiveAsFlow()
+
     val likedTrackIds: StateFlow<Set<String>> =
         likes.observeLikedTracks()
             .map { tracks -> tracks.mapTo(mutableSetOf()) { it.id } }
@@ -179,6 +188,27 @@ class PlaylistDetailViewModel @Inject constructor(
         }
     }
 
+    /**
+     * Server-side rebuild of this system playlist. No-op when the
+     * playlist isn't a refreshable system variant. On success the
+     * server rotates the playlist uuid; the new id arrives on
+     * [regenerated] for the screen to navigate-replace to.
+     */
+    fun regenerate() {
+        val playlist = (internal.value as? PlaylistDetailUiState.Success)
+            ?.detail?.playlist ?: return
+        val variant = playlist.systemVariant ?: return
+        if (!playlist.refreshable) return
+        viewModelScope.launch {
+            runCatching { repository.refreshSystemPlaylist(variant) }
+                .onSuccess { newId ->
+                    if (!newId.isNullOrEmpty()) {
+                        regeneratedChannel.trySend(newId)
+                    }
+                }
+        }
+    }
+
     /** Play the available tracks starting at [startTrackId] (or first available). */
     fun play(tracks: List<PlaylistTrackRef>, startTrackId: String?) {
         val playable = tracks.filter { it.isAvailable && !it.streamUrl.isNullOrEmpty() }
@@ -209,6 +239,16 @@ fun PlaylistDetailScreen(
     LaunchedEffect(Unit) {
         viewModel.deleted.collect { navController.popBackStack() }
     }
+    LaunchedEffect(Unit) {
+        viewModel.regenerated.collect { newId ->
+            // Replace the current detail route with the regenerated
+            // playlist's new uuid; pop the old entry so back doesn't
+            // land on a 404'd page.
+            navController.navigate(PlaylistDetail(newId)) {
+                popUpTo(PlaylistDetail(viewModel.playlistId)) { inclusive = true }
+            }
+        }
+    }
     Scaffold(
         modifier = Modifier.fillMaxSize(),
         topBar = {
@@ -241,6 +281,7 @@ fun PlaylistDetailScreen(
                         onShuffleAll = {
                             viewModel.play(s.detail.tracks.shuffled(), startTrackId = null)
                         },
+                        onRegenerate = viewModel::regenerate,
                         onTrackClick = { id ->
                             viewModel.play(s.detail.tracks, startTrackId = id)
                         },
@@ -268,6 +309,7 @@ private fun PlaylistDetailBody(
     playingTrackId: String?,
     onPlayAll: () -> Unit,
     onShuffleAll: () -> Unit,
+    onRegenerate: () -> Unit,
     onTrackClick: (String) -> Unit,
     onToggleTrackLike: (String) -> Unit,
     onNavigateToAlbum: (String) -> Unit,
@@ -277,7 +319,7 @@ private fun PlaylistDetailBody(
         modifier = Modifier.fillMaxSize(),
         contentPadding = PaddingValues(bottom = 140.dp),
     ) {
-        item { PlaylistHeader(detail.playlist, onPlayAll, onShuffleAll) }
+        item { PlaylistHeader(detail.playlist, onPlayAll, onShuffleAll, onRegenerate) }
         item { HorizontalDivider() }
         items(items = detail.tracks, key = { it.position }) { row ->
             TrackRow(
@@ -309,6 +351,7 @@ private fun PlaylistHeader(
     playlist: PlaylistRef,
     onPlayAll: () -> Unit,
     onShuffleAll: () -> Unit,
+    onRegenerate: () -> Unit,
 ) {
     Column(
         modifier = Modifier
@@ -360,6 +403,17 @@ private fun PlaylistHeader(
                 Spacer(Modifier.width(8.dp))
                 Text("Shuffle")
             }
+            if (playlist.refreshable) {
+                OutlinedButton(onClick = onRegenerate) {
+                    Icon(
+                        Lucide.RefreshCw,
+                        contentDescription = null,
+                        modifier = Modifier.size(18.dp),
+                    )
+                    Spacer(Modifier.width(8.dp))
+                    Text("Regenerate")
+                }
+            }
         }
     }
 }