Fix Slack realtime projection fallback

[snbafana]

Summary

• Fix Slack realtime projection-busy fallback so Slack queues Slack sync, not Discord sync
• Reuse a local Slack platform constant in the realtime ingest block to avoid literal drift

Verification

• pnpm test src/runtime/daemon/server.test.ts
• pnpm typecheck
• pnpm check:biome
• Push hook: pnpm check:ci-local, including pnpm build, pnpm test, and swift build --package-path native/macos/CuedNative -c release

Stack

• Base: main
• Next: codex/cued-quality-02-dead-ts-surface