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>

server/internal/tenant/domain/schedule_task.go

@@ -23,6 +23,9 @@ type ScheduleTask struct {
 	PublishAccountIDs    []string
 	CoverAssetURL        *string
 	CoverImageAssetID    *int64
+	CoverMode            string
+	CoverRandomScope     string
+	CoverRandomFolderID  *int64
 	EnableWebSearch      bool
 	GenerateCount        int
 	StartAt              *time.Time