Cameras on devices (openpanel C1): frames in memory, a binary uplink, demand

- Volatile frames are held in memory with a byte-bounded queue instead of
  the /dev/shm ring scanned on every put; workers keep writing to the
  directory, which is trimmed on a timer. A volatile upload no longer spools
  to disk. ARTIFACT_VOLATILE_BYTES=0 turns the ring off.
- A device sends camera frames as binary messages on its socket, held and
  published in one step and credited to its device node; refused frames are
  logged and dropped. Its snapshot carries demand and the uplink (binary, or
  http through the portal).
- One demand registry for every socket's wanted media, the tunnel's
  included, plus what running flows read, mapped onto on-demand ports and
  published as device_demand; at most 8 media names per socket.
- Per-message deflate off in serve and dev; a device may fetch the last 8
  digests of a message; published dashboards cached per store revision; a
  revoked device socket is closed with 1008.
- Media tiles want bytes only while on screen; the Devices tab previews a
  wired camera; the device node warns when a flow reads an on-demand port.

backend/tests/api/routes/test_flows.py

@@ -2,7 +2,6 @@ import asyncio
 from datetime import UTC, datetime
 
 import pytest
-from fastapi import WebSocketDisconnect
 from fastapi.testclient import TestClient
 from sqlalchemy import func
 from sqlmodel import Session, select
@@ -538,10 +537,10 @@ async def test_the_live_socket_ends_when_its_client_goes(
         async def send_text(self, text: str) -> None:
             self.sent.append(text)
 
-        async def receive_text(self) -> str:
+        async def receive(self) -> dict[str, object]:
             if self.frames:
-                return self.frames.pop(0)
-            raise WebSocketDisconnect(1006)
+                return {"type": "websocket.receive", "text": self.frames.pop(0)}
+            return {"type": "websocket.disconnect", "code": 1006}
 
     peer = Peer()
     subscribed = len(event_bus._subscribers)