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>
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