Add savable model default parameters to backend

[OAGr]

Description of suggestion or shortcoming:

I want to add one of these soon (https://github.com/quantified-uncertainty/squiggle/issues/3117), so I imagine we might as well add a few at once.

We can add official editing support later.

Squiggle Hub already saves the seed.

Some other ones to do:

• default_autosave_enabled: optional(boolean)
• default_sample_count: optional(number) > 0
• default_point_set_length: optional(number) > 0

Or, in schema.prisma,

autorunMode  Boolean?
sampleCount Int?
pointSetLength Int?

These would be added to SquiggleSnippet in the DB/model.

We can manually adjust these, for now.