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/app/schedule_task_policy.go

@@ -14,6 +14,10 @@ import (
 const (
 	ScheduleTargetPromptRule            = "prompt_rule"
 	ScheduleTargetKolSubscriptionPrompt = "kol_subscription_prompt"
+	ScheduleCoverModeSpecific           = "specific"
+	ScheduleCoverModeRandom             = "random"
+	ScheduleCoverRandomScopeAll         = "all"
+	ScheduleCoverRandomScopeFolder      = "folder"
 
 	scheduleKindDaily  = "daily"
 	scheduleKindWeekly = "weekly"