release(v1.7.20-alpha): stop auto-apply scheduler killing the service

The 3AM auto-update path called std::process::exit(0) immediately
after apply_update returned. apply_update had already spawned a 2s-
delayed systemctl restart, but exit(0) killed the runtime before that
spawned task could run — and the unit's Restart=on-failure does not
trigger on a clean exit 0, so the service stayed dead until someone
SSH'd in and started it manually (.253 hit this today).

Scheduler now returns from the task without killing the process;
apply_update's existing restart path (same one the UI's Install
Update button uses) brings the new version up cleanly.

Also hardens the ISO CI: the AIUI inclusion step now falls back to
extracting from the newest release tarball if the runner's cached
/opt/archipelago/web-ui/aiui path is missing, so a reprovisioned
runner can't silently ship a frontend tarball without AIUI. The ISO
build step also sanity-checks the binary exists before invoking the
builder.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

core/Cargo.lock

@@ -80,7 +80,7 @@ checksum = "a23eb6b1614318a8071c9b2521f36b424b2c83db5eb3a0fead4a6c0809af6e61"
 
 [[package]]
 name = "archipelago"
-version = "1.7.19-alpha"
+version = "1.7.20-alpha"
 dependencies = [
  "anyhow",
  "archipelago-container",