Add camera snapshot messages and flag for P1/A1 cameras

camera_snapshot_request (cloud to gateway) and camera_snapshot (gateway
to cloud) carry one JPEG frame at a time; telemetry gains camera_snapshot
and the app view gains cameraSnapshot.

schemas/continuum/v1/edge/printer_telemetry.schema.json

@@ -108,7 +108,11 @@
     },
     "camera_stream": {
       "type": "string",
-      "description": "The stream name to ask the gateway's stream_base_url for, when this printer has a camera the gateway could restream. Absent means no stream: either the printer reports no camera source, or it is a model whose camera is not reachable this way. That second case is the common one today - a Bambu P1/A1 does not publish an RTSP URL at all (confirmed: a P1P's ipcam block has ipcam_dev and ipcam_record and no rtsp_url), and only the X1/H2 class does. Those need Bambu's proprietary port-6000 JPEG stream instead, which is a separate client this gateway does not have yet."
+      "description": "The stream name to ask the gateway's stream_base_url for, when this printer has a camera the gateway could restream. Absent means no stream: either the printer reports no camera source, or it is a model whose camera is not reachable this way. That second case is the common one today - a Bambu P1/A1 does not publish an RTSP URL at all (confirmed: a P1P's ipcam block has ipcam_dev and ipcam_record and no rtsp_url), and only the X1/H2 class does. Those are served by camera_snapshot instead."
+    },
+    "camera_snapshot": {
+      "type": "boolean",
+      "description": "True when this printer's camera can only be read as single JPEG frames (a Bambu P1/A1: proprietary TLS stream on port 6000, no RTSP) and the gateway will answer a CameraSnapshotRequest for it. Mutually exclusive with camera_stream in practice. Absent means false."
     }
   },
   "required": [