Introduce an onboarding system in the Controller that guides users
through selecting and mounting their primary media drive. This is
the fallback UI flow used when the main React app is unavailable.
Key changes:
- Add onboarding API endpoints and schemas (controller/api/onboarding.py)
- Add onboarding logic to detect drives, handle already-mounted drives
(needed for RPi which auto-mounts), and save primary drive selection
to the controller config when no mounts are configured
- Extend the Controller's index.html template with the onboarding UI
- Skip root cert creation until onboarding is complete
- Update first_startup.sh to support the onboarding/repair flow
- Add comprehensive tests for onboarding logic and API
