feat(monitoring): add priority screenshot pipeline with screenshot_type + docs cleanup

Implement end-to-end support for typed screenshots and priority rendering in monitoring.

Added
- Accept and forward screenshot_type from MQTT screenshot/dashboard payloads
  (periodic, event_start, event_stop)
- Extend screenshot upload handling to persist typed screenshots and metadata
- Add dedicated priority screenshot serving endpoint with fallback behavior
- Extend monitoring overview with priority screenshot fields and summary count
- Add configurable PRIORITY_SCREENSHOT_TTL_SECONDS window for active priority state

Fixed
- Ensure screenshot cache-busting updates reliably via screenshot hash updates
- Preserve normal periodic screenshot flow while introducing event_start/event_stop priority path

Improved
- Monitoring dashboard now displays screenshot type badges
- Adaptive polling: faster refresh while priority screenshots are active
- Priority screenshot presentation is surfaced immediately to operators

Docs
- Update README and copilot-instructions to match new screenshot_type behavior,
  priority endpoint, TTL config, monitoring fields, and retention model
- Remove redundant/duplicate documentation blocks and improve troubleshooting section clarity

scheduler/db_utils.py

@@ -306,6 +306,7 @@ def format_event_with_media(event):
                 "autoplay": getattr(event, "autoplay", True),
                 "loop": getattr(event, "loop", False),
                 "volume": getattr(event, "volume", 0.8),
+                "muted": getattr(event, "muted", False),
                 # Best-effort metadata to help clients decide how to stream
                 "mime_type": mime_type,
                 "size": size,