Add a switch to turn job dispatch off, and use it in the dev stack

The scheduler sends a queued job to any idle claimed printer in any farm.
The dashboard dev stack is wired to the real printers, so testing the job
upload UI there would have started a real print on the first idle one.

DISPATCH_ENABLED=false (exactly that string) disables the timer and the
nudge from POST /jobs; any other value that looks like an attempt to
disable it is warned about, because a typo that silently leaves it on is
the dangerous direction. scripts/e2e-camera.sh now always runs the backend
with it off.

src/dispatch/index.ts

@@ -4,6 +4,7 @@ import { printers, printJobs } from "../db/schema";
 import { getDownloadUrl, headObject } from "../s3";
 import { isGatewayOnline, sendPrinterCommand } from "../routes/ws/edge";
 import { nextJobStatus } from "./job_status";
+import { dispatchEnabled, dispatchSwitchWarning } from "./enabled";
 
 /**
  * The job scheduler: picks up `queued` jobs and puts them on idle printers.
@@ -152,6 +153,8 @@ async function releaseJob(jobId: string, outcome: "requeue" | "fail", reason: st
 export { nextJobStatus } from "./job_status";
 
 export async function dispatchTick(): Promise<number> {
+  // Also covers the nudge from `POST /jobs`, which calls this directly.
+  if (!dispatchEnabled()) return 0;
   await reapStalledUploads();
 
   let dispatched = 0;
@@ -286,6 +289,12 @@ let timer: ReturnType<typeof setInterval> | undefined;
 /** Starts the scheduler. Idempotent, so a second call does not double-tick. */
 export function startDispatcher() {
   if (timer) return;
+  const warning = dispatchSwitchWarning();
+  if (warning) console.warn(`[dispatch] ${warning}`);
+  if (!dispatchEnabled()) {
+    console.log("[dispatch] DISABLED (DISPATCH_ENABLED=false): queued jobs will not be sent to printers");
+    return;
+  }
   timer = setInterval(() => {
     dispatchTick().catch((err) => console.error("[dispatch] tick failed", err));
   }, TICK_MS);