feat: 封面模板对接后端 CRUD API，移除 mock 数据

- 新增 api/cover-templates.ts：封装 GET/POST/PUT/DELETE /cover-templates
- useStep6Cover.ts：弹窗打开时 fetch 模板列表，CRUD 操作走 API
- CoverSettingsModal.tsx：增加 loading/error 状态展示 + 删除按钮（非系统模板）
- Step6CoverSettings.tsx：透传 loading/error/delete 属性
- constants.ts：删除 MOCK_COVER_TEMPLATES 硬编码数据

apps/web/src/api/cover-templates.ts

@@ -0,0 +1,53 @@
+/**
+ * 封面模板 CRUD API
+ * 后端路由: /api/v1/cover-templates
+ */
+import apiClient from "./client"
+import type { CoverTemplate } from "@/pages/editing-planner/types"
+
+export interface CoverTemplateListResponse {
+  items: CoverTemplate[]
+  total: number
+}
+
+export interface CoverTemplateCreateRequest {
+  name: string
+  config?: {
+    background_enabled?: boolean
+    background_color?: string
+    portrait_enabled?: boolean
+    title_text?: string
+    subtitle_text?: string
+    mask_enabled?: boolean
+  }
+}
+
+export type CoverTemplateUpdateRequest = Partial<CoverTemplateCreateRequest>
+
+/** 获取封面模板列表 */
+export async function fetchCoverTemplates(): Promise<CoverTemplateListResponse> {
+  const response = await apiClient.get<CoverTemplateListResponse>("/cover-templates")
+  return response.data
+}
+
+/** 创建封面模板 */
+export async function createCoverTemplate(
+  data: CoverTemplateCreateRequest,
+): Promise<CoverTemplate> {
+  const response = await apiClient.post<CoverTemplate>("/cover-templates", data)
+  return response.data
+}
+
+/** 更新封面模板 */
+export async function updateCoverTemplate(
+  id: string,
+  data: CoverTemplateUpdateRequest,
+): Promise<CoverTemplate> {
+  const response = await apiClient.put<CoverTemplate>(`/cover-templates/${id}`, data)
+  return response.data
+}
+
+/** 删除封面模板（系统模板不可删） */
+export async function deleteCoverTemplate(id: string): Promise<void> {
+  await apiClient.delete(`/cover-templates/${id}`)
+}