Native ONVIF cell-motion ingest

Adds a per-camera ONVIF subscriber that lets cameras with native
hardware motion detection (e.g. OpenIPC firmware for HiSilicon,
Ingenic and SigmaStar SoCs; many ONVIF Profile-M devices) replace
Frigate's per-frame CPU motion analysis. Two standard ONVIF
transports are consumed in parallel:

- WS-BaseNotification PullPoint for the binary motion state
  (tns1:RuleEngine/CellMotionDetector/Motion IsMotion=true|false,
   with tns1:VideoSource/MotionAlarm State=true|false accepted as
   a fallback for cameras that only publish the legacy topic).
- RTSP analytics metadata stream (application/vnd.onvif.metadata)
  for the per-frame cell grid (tt:MotionInCells, base64 + PackBits
  bit-packed bitmap). Cell layout is discovered once at startup via
  AnalyticsService.GetAnalyticsModules and the camera's CellLayout
  transformation is used to map cells to detect-frame pixel
  rectangles via connected-components.

New config:
  onvif.events.{enabled, subscription_timeout, use_metadata_stream}
  motion.source: internal (default) | onvif

When motion.source: onvif, ImprovedMotionDetector is skipped and
motion_boxes come from the camera. Internal motion remains the
default; the new path is fully opt-in.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/public/locales/en/config/global.json

@@ -777,6 +777,10 @@
       "label": "Enable motion detection",
       "description": "Enable or disable motion detection for all cameras; can be overridden per-camera."
     },
+    "source": {
+      "label": "Motion source",
+      "description": "Where motion state comes from: Frigate's internal frame analyser, or the camera's ONVIF cell-motion events (requires onvif.events.enabled)."
+    },
     "threshold": {
       "label": "Motion threshold",
       "description": "Pixel difference threshold used by the motion detector; higher values reduce sensitivity (range 1-255)."
@@ -1627,6 +1631,22 @@
         "description": "Internal field to track whether autotracking was enabled in configuration."
       }
     },
+    "events": {
+      "label": "ONVIF events",
+      "description": "Consume camera-side ONVIF motion notifications instead of Frigate's CPU motion detector.",
+      "enabled": {
+        "label": "Enable ONVIF events",
+        "description": "Subscribe to the camera's ONVIF cell-motion notifications and use them as Frigate's motion signal."
+      },
+      "subscription_timeout": {
+        "label": "Subscription timeout",
+        "description": "Seconds before the PullPoint subscription expires and is renewed."
+      },
+      "use_metadata_stream": {
+        "label": "Use metadata stream",
+        "description": "Open the ONVIF analytics RTSP metadata stream to receive per-cell motion coordinates. Falls back to a full-frame box when disabled or when the camera does not advertise the track."
+      }
+    },
     "ignore_time_mismatch": {
       "label": "Ignore time mismatch",
       "description": "Ignore time synchronization differences between camera and Frigate server for ONVIF communication."