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.

tests/dispatch.test.ts

@@ -70,3 +70,22 @@ describe("nextJobStatus", () => {
     expect(completable).toEqual(["printing", "paused"]);
   });
 });
+
+describe("the dispatch switch", () => {
+  test("is on by default and off only for the exact string false", async () => {
+    const { dispatchEnabled } = await import("../src/dispatch/enabled");
+    expect(dispatchEnabled({})).toBe(true);
+    expect(dispatchEnabled({ DISPATCH_ENABLED: "true" })).toBe(true);
+    expect(dispatchEnabled({ DISPATCH_ENABLED: "false" })).toBe(false);
+  });
+
+  test("a value that looks like an attempt to disable it but does not is reported", async () => {
+    const { dispatchEnabled, dispatchSwitchWarning } = await import("../src/dispatch/enabled");
+    for (const typo of ["0", "off", "False", "no"]) {
+      expect(dispatchEnabled({ DISPATCH_ENABLED: typo })).toBe(true);
+      expect(dispatchSwitchWarning({ DISPATCH_ENABLED: typo })).toContain("WILL be dispatched");
+    }
+    expect(dispatchSwitchWarning({})).toBeUndefined();
+    expect(dispatchSwitchWarning({ DISPATCH_ENABLED: "false" })).toBeUndefined();
+  });
+});