Reduce national forecast cache from 120s to 30s

[Gotnochill]

Closes #302

Pull Request

Description

Reduces the cache duration for the /national/forecast route from 120 seconds to 30 seconds by passing expire=30 to its @cache decorator. The global default in main.py stays at 120s; only this route is affected, as per the issue.

Fixes #302

How Has This Been Tested?

Verified the existing test suite passes with the change. The route behaviour is unchanged, and only the cache TTL is reduced.

• Yes

If your changes affect data processing, have you plotted any changes? i.e. have you done a quick sanity check?

No changes in data preprocessing.

• Yes

Checklist:

• My code follows OCF's coding style guidelines
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have checked my code and corrected any misspellings