feat: add private product management endpoints for updating canonical names and merging products

flutter/lib/core/api/api_paths.dart

@@ -10,11 +10,13 @@ class ProductApiPaths {
   static const aiCategorizeBulk = '/products/ai-categorize-bulk';
   static const deleted = '/products/deleted';
   static const merge = '/products/merge';
+  static const mergePrivate = '/products/private/merge';
   static String mergePreview(int sourceProductId, int targetProductId) =>
       '/products/merge-preview?sourceProductId=$sourceProductId&targetProductId=$targetProductId';
   static String setStatus(int id) => '/products/$id/status';
   static String update(int id) => '/products/$id';
   static String canonicalName(int id) => '/products/$id/canonical-name';
+  static String canonicalNamePrivate(int id) => '/products/private/$id/canonical-name';
   static String remove(int id) => '/products/$id';
   static String restore(int id) => '/products/$id/restore';
   static const bulkUpdate = '/products/bulk-update';

flutter/lib/features/admin/data/admin_repository.dart

@@ -191,12 +191,33 @@ class AdminRepository {
   Future<void> restoreProduct(int productId) =>
       _postVoid(ProductApiPaths.restore(productId));
 
+  // ── Product canonical name updates ────────────────────────────────────────
+  // Admin can update any product; users can only update their own private products
+  
   Future<void> updateCanonicalName(int productId, String canonicalName) =>
       _patchVoid(
         ProductApiPaths.canonicalName(productId),
         {'canonicalName': canonicalName.trim()},
       );
 
+  Future<void> updateCanonicalNamePrivate(int productId, String canonicalName) =>
+      _patchVoid(
+        ProductApiPaths.canonicalNamePrivate(productId),
+        {'canonicalName': canonicalName.trim()},
+      );
+
+  // ── Product merging ────────────────────────────────────────────────────────
+  // Admin can merge any products; users can only merge their own private products
+  
+  Future<void> mergeProductsPrivate({
+    required int sourceProductId,
+    required int targetProductId,
+  }) =>
+      _postVoid(ProductApiPaths.mergePrivate, {
+        'sourceProductId': sourceProductId,
+        'targetProductId': targetProductId,
+      });
+
   /// Skapar en ny aktiv produkt (kräver admin). Returnerar `{id, name, categoryId?}`.
   Future<Map<String, dynamic>> createProduct(String name, {int? categoryId}) =>
       _post<Map<String, dynamic>>(