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

.github/instructions/DONE.md

@@ -1161,3 +1161,24 @@ Completed the three Playout enhancement workstreams from `TODO.md`. Remaining po
 - Fixed `Uncaught TypeError: Cannot read properties of null (reading 'appendChild')` in the viewer bundle: `index.html` was missing the `#messages` chat container (and had a stray `</section>`), so `addMessage` crashed on WebSocket open/message.
 - Restored chat panel structure: added `<section id="messages" class="chat-messages">` and wrapped `#presence-list` in `<section id="presence-panel" class="presence-panel">`.
 - Made `addMessage` defensive (returns early when `#messages` is absent), matching the null-guards in `renderPresence`/`showTyping`/`hideTyping`.
+
+## Session 2026-08-13 — Chat improvements
+
+### Transient presence flash + system-message cleanup
+
+- Added `#chat-flash` container (`index.html`) + `.chat-flash` CSS (top of chat panel, `0.83rem`, 3s auto-hide with 0.5s fade-out).
+- `main.ts` `ws.onmessage` routes join/leave system messages to `showFlash()` + presence list instead of `addMessage()`.
+- Backend: `Broadcast` no longer persists `type: "system"` messages; `PushChatHistory` excludes system rows; `HandleAdminChatLog` filters `type <> 'system'`.
+- 5s reconnect grace: disconnect schedules a delayed "left" via `time.AfterFunc` (`Hub.PresenceGracePeriod`); a reconnect within grace cancels it and skips "joined".
+- Font sizing: `.message-text` set to `0.95rem` (between default and system `0.83rem`); message alignment already correct.
+- Tests: `TestHub_Broadcast_SystemNotPersisted`, `TestHub_ReconnectGracePeriod`, `TestHandleAdminChatLog_ExcludesSystem`, frontend `presence flash`; ws test DB fixed to `SetMaxOpenConns(1)`.
+
+## Session 2026-08-13 — Playout improvements
+
+### HLS segment duration 2s + window 12
+
+- `backend/playout/src/playlist.go`: `HLS_SEGMENT_DURATION` default `"6"→"2"`, `HLS_WINDOW_SIZE` `"24"→"12"`.
+- `backend/chat/src/config/runtime.go`: `defaultHLSSegmentDuration 6→2`, `defaultHLSWindowSize 24→12`.
+- ffmpeg GOP already 2s (`-g 60 -keyint_min 60` @30fps), so segments cut cleanly on keyframes — no encoder change needed.
+- Updated `runtime_test.go` defaults + env-fallback tests, `.env.example`, `docs/STREAMING.md` (2s rationale + Firefox notes), `docs/operations/cloudflare-pages-setup.md`, `docs/architecture/README.md`.
+- Verified: chat + playout `go test ./...` and `go vet ./...` all pass.

.github/instructions/TODO.md

@@ -2,6 +2,16 @@
 
 > Completed tasks moved to [DONE](./DONE.md). Remaining tasks in [BACKLOG](./BACKLOG.md).
 
+## Chat improvements (remaining)
+
+- [ ] Manual smoke: viewer join/leave shows a 3s flash (not a chat row); reconnect <5s shows no duplicate; admin chat-log table has no system rows
+
+## Playout improvements (remaining)
+
+- [ ] Test in Firefox: live playback, rebuffer/recovery, chat sync; record quirks in `docs/STREAMING.md`
+- [ ] Rebuild + run playout; confirm `playlist.m3u8` lists ~12 segments at 2s each
+- [ ] Manual: verify viewer latency and cache-hit behavior after the change
+
 ## Setup phases remaining
 
 ### Phase 5: complete manual Cloudflare and external-service setup