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;

server/migrations/20260526110000_add_publish_task_dedup_index.up.sql

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+-- Keep succeeded publish tasks in the active dedup set intentionally: the
+-- product rule is "one article can be published to an account once"; repeat
+-- publishing should use a copied/new article or an explicit admin status reset.
+CREATE UNIQUE INDEX CONCURRENTLY IF NOT EXISTS uq_desktop_tasks_publish_dedup_active
+    ON desktop_tasks (dedup_key)
+    WHERE kind = 'publish'
+      AND dedup_key IS NOT NULL
+      AND status IN ('queued', 'in_progress', 'succeeded', 'unknown');

server/migrations/20260526110001_add_publish_records_active_lookup_index.down.sql

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+DROP INDEX CONCURRENTLY IF EXISTS idx_publish_records_article_account_active;

server/migrations/20260526110001_add_publish_records_active_lookup_index.up.sql

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+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_publish_records_article_account_active
+    ON publish_records (tenant_id, article_id, platform_account_id, updated_at DESC, id DESC)
+    WHERE status IN ('queued', 'publishing', 'success');

server/migrations/20260526110002_add_desktop_tasks_publish_record_payload_index.down.sql

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+DROP INDEX CONCURRENTLY IF EXISTS idx_desktop_tasks_publish_record_payload;

server/migrations/20260526110002_add_desktop_tasks_publish_record_payload_index.up.sql

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+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_desktop_tasks_publish_record_payload
+    ON desktop_tasks (
+        tenant_id,
+        workspace_id,
+        (
+        -- Keep this expression aligned with loadExistingPublishRecordTargets.
+        CASE
+            WHEN (payload->>'publish_record_id') ~ '^[0-9]+$'
+                THEN (payload->>'publish_record_id')::bigint
+            ELSE NULL
+        END
+        )
+    )
+    WHERE kind = 'publish'
+      AND payload ? 'publish_record_id';