feat(publish-dedup): add indexes for publish task dedup and lookup

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>

server/migrations/20260526110000_add_publish_task_dedup_index.down.sql

@@ -0,0 +1 @@
+DROP INDEX CONCURRENTLY IF EXISTS uq_desktop_tasks_publish_dedup_active;