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

dashboard/src/monitoring.css

@@ -194,6 +194,32 @@
   margin-top: 0.55rem;
   font-size: 0.88rem;
   color: #64748b;
+  display: flex;
+  flex-direction: column;
+  gap: 0.35rem;
+}
+
+.monitoring-shot-type {
+  display: inline-flex;
+  align-items: center;
+  border-radius: 999px;
+  padding: 0.15rem 0.55rem;
+  font-size: 0.78rem;
+  font-weight: 700;
+}
+
+.monitoring-shot-type-periodic {
+  background: #e2e8f0;
+  color: #334155;
+}
+
+.monitoring-shot-type-event {
+  background: #ffedd5;
+  color: #9a3412;
+}
+
+.monitoring-shot-type-active {
+  box-shadow: 0 0 0 2px #fdba74;
 }
 
 .monitoring-error-box {