Change seqvars/factory default presets strategy

[holtgrewe]

Is your feature request related to a problem? Please describe.
We currently use "model clusters" library to create the factory default presets in memory only. This appeared to be a good idea as long as there were only presets and no queries. However, now we can - of course - not create foreign keys to these in memory objects 🤦.

Describe the solution you'd like

• introduce a "is factory default" boolean flag, defaulting to false and not modifiable through REST API
• create an on-startup handler that creates the presets in memory and syncs them to the database (UUIDs will remain stable)
• adjust the code for the factory defaults to use the "is factory default" boolean flag

Describe alternatives you've considered
For now, the factory defaults must be explicitely copied to per-project user modifiable presets.

Additional context
N/A