Add a unique index on desktop_tasks.dedup_key for active publish kinds
(queued/in_progress/succeeded/unknown) so duplicate publish submissions can
be rejected at the DB layer, plus supporting indexes on publish_records and
the desktop_tasks publish_record_id payload expression used by the new
dedup lookup query.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
