Filter sources without last_updated from API response

[heathdutton]

Fixes #6667

The Journalist API's /sources endpoint was returning sources that the Journalist Interface filters out. Specifically, sources with last_updated=None were included in API responses but excluded from the JI.

This adds the missing .filter(Source.last_updated.isnot(None)) to align the API with the Journalist Interface behavior.

Test plan

1. Start the dev environment and create a source that has not yet submitted anything (so last_updated is null)
2. Query /api/v1/sources - verify the incomplete source is not returned
3. Submit something as that source
4. Query /api/v1/sources again - verify the source now appears

Checklist

This change accounts for:

• any required additional documentation
• any necessary AppArmor changes (added or removed application files)
• any impact on new SecureDrop installs and upgrades
• our dependency update policy