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/features/import/domain/flyer_import_item.dart

@@ -37,7 +37,7 @@ class FlyerImportItem {
     this.matchConfidence,
   });
 
-  factory FlyerImportItem.fromJson(Map<String, dynamic> json) {
+  factory FlyerImportItem.fromJson(Map<String, dynamic> json) {
     return FlyerImportItem(
       flyerItemId: (json['flyerItemId'] as num?)?.toInt(),
       rawName: json['rawName'] as String? ?? '',
@@ -56,6 +56,28 @@ class FlyerImportItem {
       matchedProductName: json['matchedProductName'] as String?,
       matchedVia: json['matchedVia'] as String?,
       matchConfidence: (json['matchConfidence'] as num?)?.toDouble(),
-    );
-  }
-}
+    );
+  }
+
+  Map<String, dynamic> toJson() {
+    return {
+      'flyerItemId': flyerItemId,
+      'rawName': rawName,
+      'normalizedName': normalizedName,
+      'category': category,
+      'price': price,
+      'priceUnit': priceUnit,
+      'offerText': offerText,
+      'isOffer': isOffer,
+      'offerLimitText': offerLimitText,
+      'comparisonPrice': comparisonPrice,
+      'comparisonUnit': comparisonUnit,
+      'parseConfidence': parseConfidence,
+      'parseReasons': parseReasons,
+      'matchedProductId': matchedProductId,
+      'matchedProductName': matchedProductName,
+      'matchedVia': matchedVia,
+      'matchConfidence': matchConfidence,
+    };
+  }
+}

flutter/lib/features/import/domain/flyer_import_result.dart

@@ -26,4 +26,12 @@ class FlyerImportResult {
       warnings: warnings,
     );
   }
+
+  Map<String, dynamic> toJson() {
+    return {
+      'sessionId': sessionId,
+      'items': items.map((item) => item.toJson()).toList(),
+      'warnings': warnings,
+    };
+  }
 }