feat(mcp): подготовить сервер и доступ к приёмке

backend/drizzle/0013_scheduler_retention_index.sql

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+-- 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);