Improve startup lifecycle and persistence initialization

[rodja]

There are two kind of startup handlers app.on_startup which is provided by NiceGUI and rosys.on_startup from RoSys.
A few observations:

• Synchronous startup handlers are executed before asynchronous startup handlers.
• RoSys startup handlers are run after persistence is loaded
• NiceGUI startup handlers are run before persistence is loaded
• since RoSys persistence restoration is using an async startup, any setups (like cameras with calibrations) is initialized first, but then overwritten by persistence

It would be great to simplify and de-confuse this logic and provide better documentation.