fix(server,flutter): sync wire format + systemVariant column (#357)

Two fixes in one commit because they're entangled — the systemVariant
work would have been theater otherwise.

## The wire-format bug

/api/library/sync was emitting PascalCase JSON for artist / album /
track / playlist upserts (raw json.Marshal of sqlc-generated structs
with no JSON tags — sqlc.yaml: emit_json_tags=false). Flutter's
sync_controller _*FromJson reads snake_case keys, so all metadata
sync rows landed in drift with empty strings / zero ints.

The like_track / like_album / like_artist / playlist_track entities
work because they're hand-built `map[string]string` payloads with
snake_case keys — they sidestepped the bug. The 4 raw-marshal
entities did not.

Existing sync test caught zero of this — it asserts on len(upserts)
not field shape.

Fix: server-side view structs in library_sync_views.go with proper
JSON tags + pgtype-flattening (UUID → 8-4-4-4-12 hex string,
Date → "2006-01-02"). Mirrors the playlistRowView pattern from
/api/playlists. New library_sync_views_test.go pins the wire keys
so future field-name drift breaks loud.

## systemVariant column (closes #357 plan C v1 limitation)

playlistSyncView now carries `system_variant` server → wire.

Flutter drift schema bumped from 1 → 2 with onUpgrade adding the
`systemVariant TEXT NULL` column to cached_playlists. Cursor reset
to 0 in the migration so existing rows refresh with the new field
on the next sync.

playlistsListProvider now filters locally by systemVariant:
- kind='user'   → systemVariant IS NULL  (the add-to-playlist sheet's intent)
- kind='system' → systemVariant IS NOT NULL
- kind='all'    → no filter

Closes the documented v1 limitation where the add-to-playlist sheet
showed system playlists alongside user-created ones.

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

flutter_client/lib/cache/adapters.dart

@@ -88,7 +88,7 @@ extension CachedPlaylistAdapter on CachedPlaylist {
         name: name,
         description: description,
         isPublic: isPublic,
-        systemVariant: null, // not tracked in cache
+        systemVariant: systemVariant,
         trackCount: trackCount,
         coverUrl: '', // server-derived; cache doesn't persist
         ownerUsername: ownerUsername,
@@ -105,5 +105,6 @@ extension PlaylistDriftWrite on Playlist {
         description: drift.Value(description),
         isPublic: drift.Value(isPublic),
         trackCount: drift.Value(trackCount),
+        systemVariant: drift.Value(systemVariant),
       );
 }

flutter_client/lib/cache/db.dart

@@ -76,6 +76,11 @@ class CachedPlaylists extends Table {
   TextColumn get coverPath => text().nullable()();
   IntColumn get trackCount => integer().withDefault(const Constant(0))();
   IntColumn get durationSec => integer().withDefault(const Constant(0))();
+  /// Server's system_variant: null for user playlists, "for_you" /
+  /// "songs_like_artist" / "discover" for system-generated mixes.
+  /// Added in schemaVersion 2 to let the add-to-playlist sheet filter
+  /// out system playlists locally without a REST round-trip.
+  TextColumn get systemVariant => text().nullable()();
   DateTimeColumn get fetchedAt => dateTime().withDefault(currentDateAndTime)();
   @override
   Set<Column> get primaryKey => {id};
@@ -126,5 +131,22 @@ class AppDb extends _$AppDb {
   AppDb([QueryExecutor? e]) : super(e ?? driftDatabase(name: 'minstrel_cache'));
 
   @override
-  int get schemaVersion => 1;
+  int get schemaVersion => 2;
+
+  @override
+  MigrationStrategy get migration => MigrationStrategy(
+        onCreate: (m) => m.createAll(),
+        onUpgrade: (m, from, to) async {
+          if (from < 2) {
+            // Schema 2: add CachedPlaylists.systemVariant. Existing
+            // rows get null and the next sync rebuilds them with the
+            // server's system_variant value.
+            await m.addColumn(cachedPlaylists, cachedPlaylists.systemVariant);
+            // Reset cursor so the next /api/library/sync re-emits all
+            // playlists; otherwise the new column would stay null on
+            // pre-existing rows until they happen to change server-side.
+            await customStatement('UPDATE sync_metadata SET cursor = 0');
+          }
+        },
+      );
 }

flutter_client/lib/cache/sync_controller.dart

@@ -282,6 +282,7 @@ class SyncController extends AsyncNotifier<SyncResult?> {
         coverPath: drift.Value(j['cover_path'] as String?),
         trackCount: drift.Value((j['track_count'] as num?)?.toInt() ?? 0),
         durationSec: drift.Value((j['duration_sec'] as num?)?.toInt() ?? 0),
+        systemVariant: drift.Value(j['system_variant'] as String?),
       );
 }
 

flutter_client/lib/playlists/playlists_provider.dart

@@ -17,12 +17,15 @@ final playlistsApiProvider = FutureProvider<PlaylistsApi>((ref) async {
   return PlaylistsApi(await ref.watch(dioProvider.future));
 });
 
-/// Drift-first per #357 plan C. Returns all cached playlists for the
-/// current user (owned) + public playlists from others. The `kind`
-/// family arg only affects the REST cold-cache fetch shape — drift
-/// can't distinguish 'user' from 'system' kind because systemVariant
-/// isn't persisted (TODO: add column + schema bump in a follow-up).
-/// For v1 this means add-to-playlist sheet shows system playlists too.
+/// Drift-first per #357 plan C. Returns cached playlists filtered to
+/// match the `kind` family arg:
+///   - 'user' → only user-created (systemVariant null), owned by current user
+///   - 'system' → only system-generated (systemVariant non-null), owned
+///   - 'all' → everything: own user playlists + own system + others' public
+///
+/// systemVariant column added in drift schema v2 (#357 follow-up); previous
+/// behavior leaked system playlists into add-to-playlist sheet because we
+/// couldn't filter locally.
 final playlistsListProvider =
     StreamProvider.family<PlaylistsList, String>((ref, kind) {
   final db = ref.watch(appDbProvider);
@@ -42,11 +45,16 @@ final playlistsListProvider =
     },
     toResult: (rows) {
       if (user == null) return PlaylistsList.empty();
-      final owned = rows
+      final filtered = rows.where((r) {
+        if (kind == 'user') return r.systemVariant == null;
+        if (kind == 'system') return r.systemVariant != null;
+        return true; // 'all'
+      });
+      final owned = filtered
           .where((r) => r.userId == user.id)
           .map((r) => r.toRef())
           .toList();
-      final pub = rows
+      final pub = filtered
           .where((r) => r.userId != user.id && r.isPublic)
           .map((r) => r.toRef())
           .toList();