feat(ai): add three-level FAQ category management

- add FAQ category table/sql migration and initialize ddl updates

- add category service/controller with validation, default category rules, and sorting

- support faq item category binding and category-based filtering (include descendants)

- redesign FAQ page with category tree actions and UI polish

easyflow-ui-admin/app/src/locales/langs/en-US/documentCollection.json

@@ -81,6 +81,28 @@
   },
   "faq": {
     "faqList": "FAQ List",
+    "allFaq": "All FAQ",
+    "category": "Category",
+    "categoryPlaceholder": "Select a category (empty means default category)",
+    "categoryTree": "Category Tree",
+    "categoryPath": "Category Path",
+    "categoryName": "Category Name",
+    "categoryNamePlaceholder": "Please input category name",
+    "parentCategory": "Parent Category",
+    "parentCategoryPlaceholder": "Please select parent category",
+    "rootCategory": "Root Category",
+    "sortNo": "Sort No",
+    "addCategory": "Add Category",
+    "addSiblingCategory": "Add Sibling Category",
+    "addChildCategory": "Add Child Category",
+    "moveUp": "Move Up",
+    "moveDown": "Move Down",
+    "promote": "Promote",
+    "demote": "Demote",
+    "editCategory": "Edit Category",
+    "maxLevelTip": "Maximum 3 levels are supported",
+    "defaultCategoryChildForbidden": "Default category cannot have child categories",
+    "defaultCategoryDeleteForbidden": "Default category cannot be deleted",
     "question": "Question",
     "answer": "Answer",
     "questionPlaceholder": "Please input question",