infra(decommission): retire GitHub poller + streamer + */3 drain; demote D1 to hot cache

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After the serving cutover is proven, delete refresh-events.yml (the */5 poller), tear down the metagraphed-streamer Railway project, and remove the */3 R2-staging drain cron. D1 keeps only the hot recent window. Deliverable: legacy ingestion fully retired; one ingestion path (the indexer).

Part of #2108. See ADR 0013 (docs/adr/0013-hybrid-deployment-topology.md) + runbook (deploy/README.md).