fix(db): индексировать историю для очистки планировщика

backend/drizzle/0013_scheduler_retention_index.sql

@@ -0,0 +1,4 @@
+-- Retention runs after each scheduler receipt; avoid scanning the complete history.
+-- On a busy existing installation, pre-create this same index CONCURRENTLY
+-- outside a transaction before deploying (deploy/PRODUCTION-RETENTION-INDEX.md).
+CREATE INDEX IF NOT EXISTS idx_scheduler_runs_finished_at ON scheduler_runs (finished_at);