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/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?),
       );
 }