feat(flutter/cache): cacheFirst helper + drift↔model adapters (#357 plan C)

Foundation for the provider migrations. cacheFirst<D, T> wraps the
drift.watch() + REST cold-cache fallback pattern: yields cached rows
when present, kicks off REST fetch + drift populate when empty +
online, yields empty when offline. REST failures swallow to empty so
callers can surface errors via toast.

adapters.dart adds CachedX → XRef extension methods + reverse
XRef.toDrift() companions for Artist/Album/Track/Playlist. Adapters
accept some loss of server-derived fields (coverUrl, streamUrl,
ownerUsername) — UI already handles empty values; cold-cache fallback
briefly shows the real values before drift takes over.

cache_first_test covers all 4 branches (non-empty, empty+online,
empty+offline, REST failure). adapters_test covers basic round-trips.
Both safe to run on CI runner — no drift open required.

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

flutter_client/lib/cache/adapters.dart

@@ -0,0 +1,109 @@
+// Drift row → model adapters and reverse-write adapters for populating
+// drift from REST responses (#357 plan C).
+//
+// The cache loses some server-derived fields:
+//   - ArtistRef.coverUrl (server computes from most-recent album)
+//   - AlbumRef.coverUrl (server-emitted derived path)
+//   - TrackRef.streamUrl (could be reconstructed but kept empty for clarity)
+// UI already handles empty coverUrl/streamUrl gracefully. REST cold-cache
+// fallback briefly shows the real values before drift takes over.
+
+import 'package:drift/drift.dart' as drift;
+
+import '../models/album.dart';
+import '../models/artist.dart';
+import '../models/playlist.dart';
+import '../models/track.dart';
+import 'db.dart';
+
+extension CachedArtistAdapter on CachedArtist {
+  ArtistRef toRef() => ArtistRef(
+        id: id,
+        name: name,
+        sortName: sortName,
+      );
+}
+
+extension ArtistRefDriftWrite on ArtistRef {
+  CachedArtistsCompanion toDrift() => CachedArtistsCompanion.insert(
+        id: id,
+        name: name,
+        sortName: sortName.isNotEmpty ? sortName : name,
+      );
+}
+
+extension CachedAlbumAdapter on CachedAlbum {
+  /// `artistName` is supplied by the joined CachedArtists row at query time.
+  AlbumRef toRef({String artistName = ''}) => AlbumRef(
+        id: id,
+        title: title,
+        sortTitle: sortTitle,
+        artistId: artistId,
+        artistName: artistName,
+      );
+}
+
+extension AlbumRefDriftWrite on AlbumRef {
+  CachedAlbumsCompanion toDrift() => CachedAlbumsCompanion.insert(
+        id: id,
+        artistId: artistId,
+        title: title,
+        sortTitle: sortTitle.isNotEmpty ? sortTitle : title,
+      );
+}
+
+extension CachedTrackAdapter on CachedTrack {
+  /// `artistName` and `albumTitle` come from joined rows.
+  TrackRef toRef({String artistName = '', String albumTitle = ''}) => TrackRef(
+        id: id,
+        title: title,
+        albumId: albumId,
+        albumTitle: albumTitle,
+        artistId: artistId,
+        artistName: artistName,
+        trackNumber: trackNumber,
+        discNumber: discNumber,
+        durationSec: durationMs ~/ 1000,
+      );
+}
+
+extension TrackRefDriftWrite on TrackRef {
+  CachedTracksCompanion toDrift() => CachedTracksCompanion.insert(
+        id: id,
+        albumId: albumId,
+        artistId: artistId,
+        title: title,
+        durationMs: drift.Value(durationSec * 1000),
+        trackNumber: drift.Value(trackNumber),
+        discNumber: drift.Value(discNumber),
+      );
+}
+
+extension CachedPlaylistAdapter on CachedPlaylist {
+  /// `ownerUsername` is server-derived; cache stores the userId only.
+  /// Pass empty unless a join supplies it.
+  Playlist toRef({String ownerUsername = ''}) => Playlist(
+        id: id,
+        userId: userId,
+        name: name,
+        description: description,
+        isPublic: isPublic,
+        systemVariant: null, // not tracked in cache
+        trackCount: trackCount,
+        coverUrl: '', // server-derived; cache doesn't persist
+        ownerUsername: ownerUsername,
+        createdAt: '',
+        updatedAt: '',
+      );
+}
+
+extension PlaylistDriftWrite on Playlist {
+  CachedPlaylistsCompanion toDrift() => CachedPlaylistsCompanion.insert(
+        id: id,
+        userId: userId,
+        name: name,
+        description: drift.Value(description),
+        isPublic: drift.Value(isPublic),
+        trackCount: drift.Value(trackCount),
+      );
+}

flutter_client/lib/cache/cache_first.dart

@@ -0,0 +1,49 @@
+// Drift-first reactive read pattern with REST cold-cache fallback (#357 plan C).
+//
+// Subscribes to a drift watch() stream. On each emission:
+//   - non-empty → map to result type T and yield
+//   - empty + online → fetch via REST, populate drift, await re-emission
+//   - empty + offline → yield mapped empty result (UI shows empty state)
+//
+// The pattern lets every read provider trust drift as the source of
+// truth. SyncController keeps drift fresh in the background; widget
+// rebuilds happen automatically as drift writes propagate via watch().
+
+import 'dart:async';
+
+/// Wraps the watch + cold-cache fallback pattern. Generic over:
+///   D — the drift row type (or TypedResult for joins)
+///   T — the result type the caller wants (e.g. List<ArtistRef>)
+///
+/// `fetchAndPopulate` is invoked when drift is empty AND `isOnline()`
+/// returns true. It must populate drift via its own side-effect; the
+/// drift watch() stream will re-emit and this helper yields the
+/// populated rows on the next iteration.
+///
+/// REST failures are swallowed — the helper falls through to yielding
+/// the empty result. Caller is responsible for surfacing errors via
+/// toast etc.
+Stream<T> cacheFirst<D, T>({
+  required Stream<List<D>> driftStream,
+  required Future<void> Function() fetchAndPopulate,
+  required T Function(List<D>) toResult,
+  required Future<bool> Function() isOnline,
+}) async* {
+  await for (final rows in driftStream) {
+    if (rows.isNotEmpty) {
+      yield toResult(rows);
+      continue;
+    }
+    if (await isOnline()) {
+      try {
+        await fetchAndPopulate();
+        // The drift watch() stream re-emits with the populated rows on
+        // the next loop iteration. Don't yield here.
+      } catch (_) {
+        yield toResult(rows); // empty result; caller surfaces error
+      }
+    } else {
+      yield toResult(rows); // empty result; offline
+    }
+  }
+}