Remove if TYPE_CHECKING clause from most model files

[michal-lightly]

What has changed and why?

Motivation: Propagate types in schema.d.ts.

After a session with AI, I stumbled upon a solution that satisfies all three:

• avoid circular imports
• satisfy mypy
• include types in schema generation

It is possible to add imports to the bottom of the file and avoid if TYPE_CHECKING clauses. This works because:

• imports are deferred (no circular issue)
• mypy does not mind if declarations are later
• no if clause, thus types are propagated
• SQLModel/SQLAlchemy model discovery is magic that does not need the imports seemingly at all

It turns out we can likely remove all if TYPE_CHECKING clauses. This PR does it for all files except annotations (todo later).

Two other changes were needed:

• DB model is loaded before everything else. Added an import to __init__.py
• SampleTable is the first one imported

How has it been tested?

CI

Did you update CHANGELOG.md?

• Yes
• Not needed (internal change)

[michal-lightly]

/review

[horatiualmasan]

LGTM