infra(realtime): Durable Object firehose → SSE/WS + GraphQL subscriptions + MCP, plus alerter

[JSONbored]

Add a CF Durable Object fan-out hub; the indexer tees each decoded batch to it. Expose /api/v1/chain/stream (SSE) + WS with topic filters, a GraphQL Subscription type, and MCP resource subscriptions — one hub, multiple transports. Add the alerter (trigger eval + webhook/email/Telegram/Discord). Deliverable: realtime push live + alerting service.

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