feat(22-01): SSE echo-stream endpoint, edit message route, and stream tests
- Add GET /conversations/:id/stream SSE endpoint with echo-stream placeholder
- Set text/event-stream, X-Accel-Buffering: no, flush headers immediately
- Stream token events word-by-word, emit done event with messageId on completion
- Detect client disconnect via req.on(close) to stop streaming
- Persist assistant message only when stream completes (not on abort)
- Add ECHO-STREAM PLACEHOLDER comment for Phase 23 LLM adapter replacement
- Create chat-stream-routes.test.ts with SSE header, token/done event, and 404 tests
Mikkel Georgsen
fa10552127