feat(chat): add transient presence flash and 5s reconnect grace period

docs/architecture/README.md

@@ -391,7 +391,7 @@ Module-level breakdown per service.
 ### Playout & Delivery Scenario
 
 1. Admin adds normalized MP4 to R2 + updates playlist.
-2. Playout worker (FFmpeg loop with concat demuxer) reads next file, packages as HLS segments (4-6s).
+2. Playout worker (FFmpeg loop with concat demuxer) reads next file, packages as HLS segments (2s).
 3. Segments uploaded to R2 with unique immutable names; live `.m3u8` updated.
 4. Viewer loads SPA → requests master/live manifest → hls.js player follows current edge.
 5. Cloudflare serves `.m4s` and `.ts` from nearest edge cache (origin/R2 hit only once per POP/region).