[Beginner]: Extract `countries` fallback list in `fetch_jobspy_jobs` to a constant

Description

The list of countries [{'code': 'USA'...}, ...] is hardcoded as a fallback in fetch_jobspy_jobs. Move this to a top-level constant DEFAULT_JOBSPY_COUNTRIES.

Why it matters

Configuration defaults shouldn't be hidden arbitrarily inside long functions.

Acceptance Criteria

• [ ] Extract the fallback list to DEFAULT_JOBSPY_COUNTRIES
• [ ] Reference the constant inside the get() fallback
• [ ] Passes pre-commit run --all-files