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

CHANGELOG.md

@@ -4,6 +4,23 @@
 
 ## [Unreleased]
 
+### Added
+
+- **Transient presence flash**: Join/leave announcements now appear as a 3-second flash at the top of the chat panel (`#chat-flash`) with a fade-out, instead of permanent chat rows. Presence indicators still update from the same events.
+
+### Changed
+
+- **System messages no longer persisted**: The chat backend no longer stores `system`-type messages; chat history and the admin chat-log table only contain real chat messages (moderation actions remain in the audit log).
+- **Reconnect grace period**: Disconnects schedule a delayed "left" announcement (5s); a reconnect within 5s cancels it and skips "joined", eliminating join/leave spam on brief reconnects.
+- **HLS segment duration 2s**: Default `HLS_SEGMENT_DURATION` changed from 6 to 2 and `HLS_WINDOW_SIZE` from 24 to 12 (2s × 12 = 24s live window), reducing end-to-end latency; the 2s GOP was already in place.
+- **Chat font sizing**: `.message-text` set to `0.95rem` (slightly smaller than the default, larger than the `0.83rem` system/flash text).
+
+### Tests
+
+- WebSocket: `TestHub_Broadcast_SystemNotPersisted`, `TestHub_ReconnectGracePeriod`.
+- Handlers: `TestHandleAdminChatLog_ExcludesSystem`.
+- Frontend: `presence flash` test.
+
 ## [1.0.13] - 2026-08-13
 
 ### Added