feat(flyer-import): add session management and retrieval endpoints

- Add new API endpoints for retrieving flyer import sessions:
  - GET /flyer-import/sessions/latest - Retrieve latest session for user
  - GET /flyer-import/sessions/:sessionId - Retrieve specific session
- Implement session persistence and restoration in Flutter UI
- Add toJson() methods to FlyerImportItem and FlyerImportResult for serialization
- Add new FlyerImportSession domain model for local session management
- Add unit test file for FlyerImportService
- Update FlyerImportController with new endpoints and user ID extraction
- Update FlyerImportService with session retrieval logic and response mapping
- Update API paths in Flutter client
- Add session restoration on widget init in FlyerImportTab

flutter/lib/core/api/api_paths.dart

@@ -40,6 +40,8 @@ class ReceiptImportApiPaths {
 
 class FlyerImportApiPaths {
   static const parse = '/flyer-import/parse';
+  static const latestSession = '/flyer-import/sessions/latest';
+  static String bySession(int sessionId) => '/flyer-import/sessions/$sessionId';
 }
 
 class FlyerSelectionApiPaths {