feat(inventory): CSV/JSON export and JSON import with LWW reconciliation

Interchange export/import for Phase 1 (data-model §7):

- JSON: canonical, versioned envelope (formatVersion 1) with all sync
  metadata verbatim, photos embedded as base64, tombstones excluded.
  Species are re-resolved on import by scientific name (catalog ids are
  per-install). Reader tolerates unknown fields/enum values (§5.2) and
  rejects newer format versions with a clear error.
- CSV: export-only spreadsheet flatten, one row per lot, RFC 4180
  escaping, never any photo bytes.
- Import merges by UUIDv7 id in one transaction: insert-if-unknown
  preserving original stamps, last-writer-wins by packed HLC for known
  mutable rows, append-only movements; afterwards the local clock
  receiveEvent()s the newest imported stamp so it never runs behind.
- Settings gains a Backup section (export CSV/JSON, import JSON with
  confirmation); file dialogs behind a FileService interface backed by
  file_picker (MIT).
- Tests: codec round-trip and tolerance, reconciler LWW, repository
  export→import round-trip (fresh DB, idempotent re-import, newer-local
  wins, clock monotonicity, species re-resolution), backup widget flows.

apps/app_seeds/lib/db/database.dart

@@ -26,8 +26,12 @@ part 'database.g.dart';
 class AppDatabase extends _$AppDatabase {
   AppDatabase(super.e);
 
+  /// Current schema version; also stamped into interchange exports so an
+  /// importer knows which app generation wrote the file (data-model §7).
+  static const int currentSchemaVersion = 5;
+
   @override
-  int get schemaVersion => 5;
+  int get schemaVersion => currentSchemaVersion;
 
   @override
   MigrationStrategy get migration => MigrationStrategy(