perf(chat): unblock response delivery and measure send phases

[tannerlinsley]

Transcript updates can wait behind a slow sidebar activity projection, even after model output is available. Give each durable outbox its own coalesced publisher, so sidebar PostgreSQL work does not hold up the response stream. Overlap the independent HTTP workflow, lifecycle, and run-context reads after conversation authorization.

Add content-free phase timings for HTTP admission, durable preparation, usage reservation, enrichment, model context, and first text. A live trace measured 1,398 ms inside admission, but the remaining time has not yet been attributed precisely. These logs allow that attribution without treating browser observation time as database latency.

Validation: full pnpm test passed, including all type checks, lint, 537 site tests, 2,274 chat tests, and 20 desktop tests. The real PostgreSQL runtime suite completed successfully. The focused publication and run tests passed 17 cases, including a new regression that blocks sidebar publication while later transcript updates continue. Durable receipts, publisher coalescing, retries, and copy activation checks remain intact.

Summary by CodeRabbit

• Bug Fixes
  • Conversation transcript updates can now continue while bot-activity updates are delayed, reducing the impact of slow activity subscribers.
  • Send requests perform independent data reads concurrently, which may reduce response time while preserving access and lifecycle checks.
• Improvements
  • Added structured timing diagnostics for conversation sends and runs, including first-response timing, without recording message text or credentials.

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• harness-tests/pending-runtime/conversation-runs-runtime.test.ts
• src/chat/server/conversation-http.server.ts
• src/chat/server/conversation.ts

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