feat(flutter/cache): migrate artistProvider to drift-first (#357 plan C)

artistProvider becomes a StreamProvider backed by drift's cached_artists
watch(). Empty + online triggers REST fallback; populates drift; watch
re-emits. Empty + offline yields a sentinel ArtistRef(id: '', name: '');
surfaces should already check state.value?.id.isNotEmpty.

AsyncValue<ArtistRef> shape unchanged so .when callsites keep working.

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

flutter_client/lib/library/library_providers.dart

@@ -1,9 +1,15 @@
 import 'package:dio/dio.dart';
+import 'package:drift/drift.dart' as drift;
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import '../api/client.dart';
 import '../api/endpoints/library.dart';
 import '../auth/auth_provider.dart';
+import '../cache/adapters.dart';
+import '../cache/audio_cache_manager.dart' show appDbProvider;
+import '../cache/cache_first.dart';
+import '../cache/connectivity_provider.dart';
+import '../cache/db.dart';
 import '../models/album.dart';
 import '../models/artist.dart';
 import '../models/home_data.dart';
@@ -37,9 +43,26 @@ final homeProvider = FutureProvider<HomeData>((ref) async {
   return (await ref.watch(libraryApiProvider.future)).getHome();
 });
 
+/// Drift-first per #357 plan C. Watches cached_artists for the row;
+/// when empty + online, fetches via REST + populates drift, which
+/// re-emits via watch().
 final artistProvider =
-    FutureProvider.family<ArtistRef, String>((ref, id) async {
-  return (await ref.watch(libraryApiProvider.future)).getArtist(id);
+    StreamProvider.family<ArtistRef, String>((ref, id) {
+  final db = ref.watch(appDbProvider);
+  return cacheFirst<CachedArtist, ArtistRef>(
+    driftStream: (db.select(db.cachedArtists)..where((t) => t.id.equals(id)))
+        .watch(),
+    fetchAndPopulate: () async {
+      final api = await ref.read(libraryApiProvider.future);
+      final fresh = await api.getArtist(id);
+      await db.into(db.cachedArtists).insertOnConflictUpdate(fresh.toDrift());
+    },
+    toResult: (rows) => rows.isEmpty
+        ? const ArtistRef(id: '', name: '')
+        : rows.first.toRef(),
+    isOnline: () async =>
+        (await ref.read(connectivityProvider.future)),
+  );
 });
 
 final artistAlbumsProvider =