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/archipelago/src/update.rs

@@ -882,9 +882,15 @@ pub async fn run_update_scheduler(data_dir: std::path::PathBuf) {
                             debug!("Update scheduler: apply failed: {}", e);
                             continue;
                         }
-                        info!("Update scheduler: update applied, restart needed");
-                        // Signal for service restart (systemd will handle via exit code)
-                        std::process::exit(0);
+                        info!("Update scheduler: update applied, restart scheduled by apply_update");
+                        // apply_update has already spawned a 2s-delayed
+                        // `systemctl restart archipelago`. Don't call
+                        // std::process::exit here — that kills the runtime
+                        // before the spawned restart task runs, and since
+                        // the unit is Restart=on-failure a clean exit(0)
+                        // leaves the service dead. Fall through; the
+                        // scheduled restart will bring us back cleanly.
+                        return;
                     }
                     Ok(_) => {
                         debug!("Update scheduler: no update available");