Add client volume settings and harden screenshot publishing

- persist client config from MQTT for display manager volume control
- apply effective VLC volume from event volume and client multiplier
- support live runtime volume updates for active video playback
- make latest screenshot handoff atomic to avoid broken latest.jpg races
- ignore broken screenshot pointers when selecting fallback images
- fix screenshot size logging and document server-side volume control

README.md

@@ -98,6 +98,7 @@ CEC_DEVICE=0                     # Target device (0 recommended for TV)
 CEC_TURN_OFF_DELAY=30            # Seconds to wait before turning off TV
 CEC_POWER_ON_WAIT=5              # Seconds to wait after power ON (for TV boot)
 CEC_POWER_OFF_WAIT=5             # Seconds to wait after power OFF
+
 ```
 
 ### 3. Start Services
@@ -217,7 +218,10 @@ Notes:
   - `autoplay` (boolean): start playback automatically when the event becomes active (default: true).
   - `loop` (boolean): loop playback indefinitely.
   - `volume` (float): 0.0–1.0 (mapped internally to VLC's 0–100 volume scale).
+- Effective playback volume is calculated as `event.video.volume * client_config.audio.video_volume_multiplier` and then mapped to VLC's 0–100 scale. Example: `volume: 0.8` with `audio.video_volume_multiplier: 0.5` results in 40% VLC volume.
 - If `python-vlc` is not installed, the Display Manager will fall back to launching the external `vlc` binary.
+- HDMI-CEC remains the recommended mechanism for TV power control only. TV volume via CEC is not implemented because support is device-dependent and much less reliable than controlling VLC directly.
+- The client-wide multiplier is intended to be sent over the existing MQTT config topic `infoscreen/{client_id}/config` and is persisted locally in `src/config/client_settings.json` for the Display Manager.
 - 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.