feat(client-monitoring): finalize client-side monitoring and UTC logging

- add process health bridge and monitoring flow between display_manager and simclient
- publish health + warn/error log topics over MQTT
- standardize log/payload/screenshot timestamps to UTC (Z) to avoid DST drift
- improve video handling: python-vlc fullscreen enforcement and runtime PID reporting
- update README and copilot instructions with monitoring architecture and troubleshooting
- add Phase 3 monitoring implementation documentation
- update gitignore for new runtime/log artifacts

README.md

@@ -11,6 +11,7 @@ Digital signage system for Raspberry Pi that displays presentations, videos, and
 - **MQTT Integration** - Real-time event management from central server
 - **Group Management** - Organize clients into groups for targeted content
 - **Heartbeat Monitoring** - Regular status updates and screenshot dashboard
+- **Client Process Monitoring** - Health-state bridge, crash/restart tracking, and monitoring log
 - **Screenshot Dashboard** - Automatic screen capture with Wayland/X11 support, client-side compression
 - **Multi-Content Support** - Presentations, videos, and web pages
 - **Kiosk Mode** - Full-screen display with automatic startup
@@ -217,7 +218,13 @@ Notes:
   - `loop` (boolean): loop playback indefinitely.
   - `volume` (float): 0.0–1.0 (mapped internally to VLC's 0–100 volume scale).
 - If `python-vlc` is not installed, the Display Manager will fall back to launching the external `vlc` binary.
-- The manager attempts to make the player window fullscreen and remove window decorations. For a truly panel-free fullscreen (no taskbar), run the Display Manager inside a minimal kiosk X session or a dedicated user session without a desktop panel (see the kiosk notes below).
+- Fullscreen behavior:
+  - External VLC fallback uses `--fullscreen`.
+  - `python-vlc` mode enforces fullscreen on startup and retries fullscreen toggling briefly because video outputs may attach asynchronously.
+  - For a truly panel-free fullscreen (no taskbar), run the Display Manager inside a minimal kiosk X session or a dedicated user session without a desktop panel.
+- Monitoring PID behavior:
+  - External VLC fallback reports the external `vlc` process PID.
+  - `python-vlc` mode is in-process, so monitoring reports the `display_manager.py` runtime PID.
 
 ## 🌐 Web Events
 
@@ -380,6 +387,9 @@ The MQTT client ([src/simclient.py](src/simclient.py)) downloads presentation fi
 - `infoscreen/discovery` - Initial client announcement
 - `infoscreen/{client_id}/heartbeat` - Regular status updates
 - `infoscreen/{client_id}/dashboard` - Screenshot images (base64)
+- `infoscreen/{client_id}/health` - Process health state (`event_id`, process, pid, status)
+- `infoscreen/{client_id}/logs/error` - Forwarded client error logs
+- `infoscreen/{client_id}/logs/warn` - Forwarded client warning logs
 
 #### Server → Client
 - `infoscreen/{client_id}/discovery_ack` - Server response with client ID
@@ -493,6 +503,30 @@ This is the fastest workaround if hardware decode is not required or not availab
 ./scripts/test-mqtt.sh
 ```
 
+### Monitoring and UTC timestamps
+
+Client-side monitoring is implemented with a health-state bridge between `display_manager.py` and `simclient.py`.
+
+- Health bridge file: `src/current_process_health.json`
+- Local monitoring log: `logs/monitoring.log`
+- Process states: `running`, `crashed`, `stopped`
+- Restart tracking: bounded restart attempts per active event
+
+UTC timestamp policy:
+
+- `display_manager.log`, `simclient.log`, and `monitoring.log` are written in UTC (`...Z`)
+- MQTT payload timestamps (heartbeat/dashboard/health/log messages) are UTC ISO timestamps
+- Screenshot metadata timestamps are UTC ISO timestamps
+
+This prevents daylight-saving and timezone drift issues across clients.
+
+### VLC/PulseAudio warnings in remote sessions
+
+Warnings such as `pulse audio output error: overflow, flushing` can appear when testing through remote desktop/audio forwarding (for example, NoMachine) or virtual/dummy display setups.
+
+- If playback and audio are stable on a real HDMI display, this is usually non-fatal.
+- If warnings appear only in remote sessions, treat them as environment-related rather than a core video playback bug.
+
 ### Screenshots not uploading
 
 **Check which session type you're running:**