Relay P1/A1 camera frames to viewers

GET /printers/:id/snapshot asks the farm's gateway for one JPEG frame
(camera_snapshot_request) and returns it no-store. Concurrent viewers
share one read and a frame under 800 ms old is reused, because the
printer's camera serves one client. /stream answers kind "snapshot" for
these printers; printers.camera_snapshot is set from telemetry.

scripts/e2e-camera-snapshot.sh reads real frames from the P1P through
the real proxy and backend.

drizzle/meta/_journal.json

@@ -36,6 +36,13 @@
       "when": 1793285676878,
       "tag": "0004_printer_alerts",
       "breakpoints": true
+    },
+    {
+      "idx": 5,
+      "version": "7",
+      "when": 1793289367708,
+      "tag": "0005_bouncy_cannonball",
+      "breakpoints": true
     }
   ]
 }