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.

docker/compose.dev.yml

@@ -94,19 +94,24 @@ services:
     build:
       context: ..
       dockerfile: backend/Dockerfile
-    # `fastapi dev`, because `run` takes no `--reload-dir`. Watching the worker
+    # uvicorn itself, because `fastapi dev` cannot turn per-message deflate
+    # off, which `fluksio serve` does for the media frames. Watching the worker
     # too makes a worker edit restart the engine, which respawns the local
     # worker pool with the new code.
     command:
-      - fastapi
-      - dev
+      - uvicorn
+      - fluksio.main:app
+      - --reload
       - --host
       - 0.0.0.0
+      - --port
+      - "8000"
+      - --ws-per-message-deflate
+      - "false"
       - --reload-dir
       - /app/backend
       - --reload-dir
       - /app/worker
-      - "fluksio/main.py"
     develop:
       watch:
         - path: ../backend