feat(backend): 新增商品属性字典表（产地/保质期/储存方式/描述文档）

- model/product_attr.go: 新增 4 张字典表 struct（TenantBase 范式）
- model/product.go: Product 增加 4 个可空外键 + belongs-to 关联
- handler/product_attr.go: 4 组 16 个 CRUD handler（shop_id 隔离）
- handler/product.go: FindOrCreate/Update 支持写入 4 个属性 ID
- handler/public.go: Preload 4 表 + 三级回退介绍 + 默认话术常量
- router/router.go: /product-options/{origins,shelf-lives,storages,description-docs} 路由
- main.go: AutoMigrate 注册 4 个新 model
- testutil/setup.go: SQLite 测试库补齐新列与新表

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/testutil/setup.go

@@ -124,7 +124,51 @@ func SetupTestDB() *gorm.DB {
 			custom_fields TEXT,
 			remark TEXT,
 			name_pinyin TEXT DEFAULT '',
-			name_initials TEXT DEFAULT ''
+			name_initials TEXT DEFAULT '',
+			origin_id INTEGER,
+			shelf_life_id INTEGER,
+			storage_id INTEGER,
+			description_doc_id INTEGER
+		)`,
+		`CREATE TABLE IF NOT EXISTS product_origin_options (
+			id INTEGER PRIMARY KEY AUTOINCREMENT,
+			created_at DATETIME,
+			updated_at DATETIME,
+			deleted_at DATETIME,
+			shop_id INTEGER NOT NULL,
+			code TEXT,
+			name TEXT NOT NULL,
+			remark TEXT
+		)`,
+		`CREATE TABLE IF NOT EXISTS product_shelf_life_options (
+			id INTEGER PRIMARY KEY AUTOINCREMENT,
+			created_at DATETIME,
+			updated_at DATETIME,
+			deleted_at DATETIME,
+			shop_id INTEGER NOT NULL,
+			code TEXT,
+			name TEXT NOT NULL,
+			remark TEXT
+		)`,
+		`CREATE TABLE IF NOT EXISTS product_storage_options (
+			id INTEGER PRIMARY KEY AUTOINCREMENT,
+			created_at DATETIME,
+			updated_at DATETIME,
+			deleted_at DATETIME,
+			shop_id INTEGER NOT NULL,
+			code TEXT,
+			name TEXT NOT NULL,
+			remark TEXT
+		)`,
+		`CREATE TABLE IF NOT EXISTS product_description_docs (
+			id INTEGER PRIMARY KEY AUTOINCREMENT,
+			created_at DATETIME,
+			updated_at DATETIME,
+			deleted_at DATETIME,
+			shop_id INTEGER NOT NULL,
+			title TEXT NOT NULL,
+			content TEXT,
+			remark TEXT
 		)`,
 		`CREATE TABLE IF NOT EXISTS warehouses (
 			id INTEGER PRIMARY KEY AUTOINCREMENT,