feat(schedule): support random cover source for scheduled and manual publish

Add a cover mode to schedule tasks so auto-publish can pick a random cover
image instead of a fixed one, scoped to all images or a specific folder.

- migration: add cover_mode / cover_random_scope / cover_random_folder_id
  columns and check constraints on schedule_tasks
- backend: validate cover mode/scope/folder on create & update; the dispatch
  worker resolves a random active image (signed asset URL) at enqueue time
- frontend: new CoverSourceSelector component wired into GenerateTaskDrawer
  and PublishArticleModal, plus coverSource i18n strings

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/shared-types/src/index.ts

@@ -2132,6 +2132,8 @@ export type ScheduleTargetType = 'prompt_rule' | 'kol_subscription_prompt'
 export type ScheduleKind = 'daily' | 'weekly'
 export type ScheduleTimeMode = 'fixed' | 'random_window'
 export type ScheduleDispatchStatus = 'success' | 'failed' | string
+export type ScheduleCoverMode = 'specific' | 'random'
+export type ScheduleCoverRandomScope = 'all' | 'folder'
 
 export interface ScheduleTask {
   id: number
@@ -2159,6 +2161,9 @@ export interface ScheduleTask {
   auto_publish_platforms: string[]
   cover_asset_url: string | null
   cover_image_asset_id: number | null
+  cover_mode: ScheduleCoverMode
+  cover_random_scope: ScheduleCoverRandomScope
+  cover_random_folder_id: number | null
   enable_web_search: boolean
   generate_count: number
   start_at: string | null
@@ -2194,6 +2199,9 @@ export interface ScheduleTaskRequest {
   publish_enterprise_site_targets?: ScheduleEnterpriseSiteTarget[]
   cover_asset_url?: string | null
   cover_image_asset_id?: number | null
+  cover_mode?: ScheduleCoverMode
+  cover_random_scope?: ScheduleCoverRandomScope
+  cover_random_folder_id?: number | null
   enable_web_search?: boolean
   generate_count?: number
   start_at?: string