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

sql/06-easyflow-v2.p3-faq-category.sql

@@ -0,0 +1,62 @@
+SET NAMES utf8mb4;
+SET FOREIGN_KEY_CHECKS = 0;
+
+-- P3: FAQ分类树增强（最多三级）
+
+CREATE TABLE IF NOT EXISTS tb_faq_category
+(
+    id            bigint UNSIGNED NOT NULL COMMENT '主键',
+    collection_id bigint UNSIGNED NOT NULL COMMENT '知识库ID',
+    parent_id     bigint UNSIGNED NOT NULL DEFAULT 0 COMMENT '父分类ID，0表示根',
+    ancestors     varchar(512) NOT NULL DEFAULT '0' COMMENT '祖先路径(逗号分隔)',
+    level_no      tinyint UNSIGNED NOT NULL DEFAULT 1 COMMENT '层级(1-3)',
+    category_name varchar(64) NOT NULL COMMENT '分类名称',
+    sort_no       int NOT NULL DEFAULT 0 COMMENT '排序',
+    is_default    tinyint(1) NOT NULL DEFAULT 0 COMMENT '是否默认分类',
+    status        int NOT NULL DEFAULT 0 COMMENT '数据状态',
+    created       datetime NULL COMMENT '创建时间',
+    created_by    bigint UNSIGNED NULL COMMENT '创建人',
+    modified      datetime NULL COMMENT '更新时间',
+    modified_by   bigint UNSIGNED NULL COMMENT '更新人',
+    PRIMARY KEY (id),
+    INDEX idx_faq_category_collection_parent_sort (collection_id, parent_id, sort_no),
+    INDEX idx_faq_category_collection_level (collection_id, level_no),
+    INDEX idx_faq_category_collection_status (collection_id, status)
+) ENGINE = InnoDB DEFAULT CHARSET = utf8mb4 COLLATE = utf8mb4_0900_ai_ci COMMENT ='FAQ分类';
+
+-- 兼容低版本 MySQL：不使用 ADD COLUMN / ADD INDEX IF NOT EXISTS
+SET @faq_item_category_col_exists := (
+    SELECT COUNT(1)
+    FROM information_schema.COLUMNS
+    WHERE TABLE_SCHEMA = DATABASE()
+      AND TABLE_NAME = 'tb_faq_item'
+      AND COLUMN_NAME = 'category_id'
+);
+
+SET @faq_item_add_category_col_sql := IF(
+        @faq_item_category_col_exists = 0,
+        'ALTER TABLE tb_faq_item ADD COLUMN category_id bigint UNSIGNED NULL DEFAULT NULL COMMENT ''FAQ分类ID'' AFTER collection_id',
+        'DO 0'
+                                   );
+PREPARE stmt_add_faq_item_category_col FROM @faq_item_add_category_col_sql;
+EXECUTE stmt_add_faq_item_category_col;
+DEALLOCATE PREPARE stmt_add_faq_item_category_col;
+
+SET @faq_item_category_idx_exists := (
+    SELECT COUNT(1)
+    FROM information_schema.STATISTICS
+    WHERE TABLE_SCHEMA = DATABASE()
+      AND TABLE_NAME = 'tb_faq_item'
+      AND INDEX_NAME = 'idx_faq_collection_category_order'
+);
+
+SET @faq_item_add_category_idx_sql := IF(
+        @faq_item_category_idx_exists = 0,
+        'ALTER TABLE tb_faq_item ADD INDEX idx_faq_collection_category_order (collection_id, category_id, order_no)',
+        'DO 0'
+                                   );
+PREPARE stmt_add_faq_item_category_idx FROM @faq_item_add_category_idx_sql;
+EXECUTE stmt_add_faq_item_category_idx;
+DEALLOCATE PREPARE stmt_add_faq_item_category_idx;
+
+SET FOREIGN_KEY_CHECKS = 1;