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/app/telemetry_message.schema.json

@@ -62,6 +62,10 @@
           "type": ["string", "null"],
           "description": "The go2rtc stream name, when this printer has a camera that can actually be restreamed. Null is the common case - only Bambu's X1/H2 class publishes an RTSP URL. Present means it is worth calling GET /printers/:id/stream; null means render no player."
         },
+        "cameraSnapshot": {
+          "type": "boolean",
+          "description": "True when the camera can only be read as refreshing JPEG frames (Bambu P1/A1); GET /printers/:id/stream then answers kind 'snapshot'."
+        },
         "lastSeenAt": {
           "type": ["string", "null"],
           "format": "date-time",