fix(updater): fail health check fast, signal busy during boot provisioning, keep overlay through UI restart

[gmmcosta15]

Description

• Feature
• Bug fix
• Code refactor
• Documentation

Fixes the double Spoolman start, the loading-screen flash on update, and the boot-time Spoolman install.

• Fail-fast Spoolman health check, no double start, overlay held until fresh status.
• Skip the background apt upgrade when a daemon restart is pending, so the restart can't SIGKILL dpkg.
• install-updater.sh enables BlocksScreen.service, so the UI starts at boot.
• Boot provisioning: busy is set before the D-Bus export, so the overlay shows before MainWindow, and provisioning retries every 3 s (up to 10 times) while boot reconcile holds the lock.
• The overlay reads "Missing component, installing ..." then "Installing : ".
• replay_busy() re-emits a busy=True that fired before MainWindow wired its slots.

Tests

• Unit tests added for each change.
• On an RF50, Release-2.0 -> this branch:
  • Spoolman installs and health returns 200.
  • The UI recovers.
  • Boot with Spoolman removed shows the overlay first, installs it, and comes up healthy.
  • The apt skip was seen in the journal.