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.

.env.example

@@ -53,3 +53,8 @@ SUPABASE_URL=
 # refuses to start without it, because anything that knows this secret can
 # connect as any farm's gateway. Generate one with: openssl rand -hex 32
 EDGE_JWT_SECRET=
+
+# The scheduler sends queued jobs to any idle, claimed printer in any farm.
+# Set to "false" (exactly) on a stack wired to real printers that should not
+# start prints - scripts/e2e-camera.sh does this for you. Unset = on.
+# DISPATCH_ENABLED=false