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.

frontend/src/components/Flow/FlowEditor.tsx

@@ -511,6 +511,19 @@ function FlowEditorInner({
     }
   }, [key])
 
+  // What the flow's nodes read, for a device node to say that a camera read
+  // here sends all the time.
+  const readers = useMemo(() => {
+    const found = new Set<string>()
+    for (const node of definitions) {
+      if (node.type === DEVICE_TYPE) continue
+      for (const spec of node.requires ?? []) {
+        if (spec.name) found.add(qualify(flowName, spec.name))
+      }
+    }
+    return found
+  }, [definitions, flowName])
+
   /**
    * Where clicking an endpoint takes you: the thing it stands for.
    *
@@ -1508,6 +1521,7 @@ function FlowEditorInner({
         nodeTypes={nodeTypeInfo ?? []}
         origin={flowDoc.origin}
         suggestions={suggestions}
+        readers={readers}
         expanded={editorExpanded}
         onToggleExpand={() => setEditorExpanded((wide) => !wide)}
         onChange={updateNode}

frontend/src/components/Flow/NodePanel.tsx

@@ -56,6 +56,7 @@ import useAuth from "@/hooks/useAuth"
 import useCustomToast from "@/hooks/useCustomToast"
 import { inCodeEditor, useShortcuts } from "@/lib/shortcuts"
 import { cn } from "@/lib/utils"
+import { qualify } from "./deriveEdges"
 import { DtypeValue } from "./FlowBoundary"
 import {
   IDENTIFIER,
@@ -508,11 +509,14 @@ function DeviceSettings({
   node,
   flow,
   suggestions,
+  readers,
   onChange,
 }: {
   node: NodeDef_Input
   flow: string
   suggestions: PortSuggestions
+  /** Qualified messages another node of this flow reads. */
+  readers: ReadonlySet<string>
   onChange: (next: NodeDef_Input) => void
 }) {
   const { data } = useQuery(devicesQueryOptions())
@@ -591,6 +595,7 @@ function DeviceSettings({
         flow={flow}
         config={config}
         reported={reported}
+        readers={readers}
         suggestions={suggestions.provides}
         onChange={ports("provides")}
         onConfig={setConfig}
@@ -619,6 +624,7 @@ function DevicePortList({
   flow,
   config,
   reported,
+  readers,
   suggestions,
   onChange,
   onConfig,
@@ -632,6 +638,8 @@ function DevicePortList({
   config: Record<string, unknown>
   /** What the device says it has, or null when it has not said. */
   reported: ReportedPort[] | null
+  /** Qualified messages another node of this flow reads. */
+  readers?: ReadonlySet<string>
   suggestions: string[]
   /** `renamed` is a port taking another name, or dropped (`to` of null). */
   onChange: (next: MessageSpec[], renamed?: [string, string | null]) => void
@@ -809,6 +817,18 @@ function DevicePortList({
                 The device does not report this port.
               </p>
             ) : null}
+            {said?.on_demand &&
+            spec.name &&
+            readers?.has(qualify(flow, spec.name)) ? (
+              <p
+                className="pl-5.5 text-xs text-status-warning"
+                data-testid="device-port-always-on"
+              >
+                A node in this flow reads every frame, so {port} sends all the
+                time, also while nobody watches. Read snapshot or person
+                instead.
+              </p>
+            ) : null}
             {schema ? (
               <div className="pl-5.5">
                 <ParamsForm
@@ -1771,6 +1791,7 @@ function PanelBody({
   nodeType,
   origin,
   suggestions,
+  readers,
   expanded,
   onChange,
   onRenameMessage,
@@ -1783,6 +1804,7 @@ function PanelBody({
   nodeType: NodeTypeInfo | undefined
   origin: FlowOrigin | null | undefined
   suggestions: PortSuggestions
+  readers: ReadonlySet<string>
   expanded: boolean
   onChange: (next: NodeDef_Input) => void
   onRenameMessage: (previous: string, next: string) => void
@@ -1954,6 +1976,7 @@ function PanelBody({
             node={node}
             flow={flow}
             suggestions={suggestions}
+            readers={readers}
             onChange={editNode}
           />
         ) : (
@@ -2133,6 +2156,7 @@ export function NodePanel({
   nodeTypes,
   origin,
   suggestions,
+  readers,
   expanded,
   onChange,
   onRenameMessage,
@@ -2148,6 +2172,8 @@ export function NodePanel({
   /** Set when the flow was declared in code elsewhere; its bodies are generated. */
   origin: FlowOrigin | null | undefined
   suggestions: PortSuggestions
+  /** Qualified messages the flow's nodes read, a device node's aside. */
+  readers: ReadonlySet<string>
   expanded: boolean
   onChange: (next: NodeDef_Input) => void
   onRenameMessage: (previous: string, next: string) => void
@@ -2201,6 +2227,7 @@ export function NodePanel({
           nodeType={nodeType}
           origin={origin}
           suggestions={suggestions}
+          readers={readers}
           expanded={expanded}
           onChange={onChange}
           onRenameMessage={onRenameMessage}