add client groups API route, fetch script, and unit test

[james-cross]

• Added /api/client-groups serverless API route to return all client groups from the database.
• Created a new fetch-client-groups.js script to generate a static client-groups.json file for local use.
• Refactored route logic into a reusable getClientGroups helper for easier unit testing.
• Added Jest unit test for the helper to verify database output and error handling.
• Verified that clientGroups are displayed on Find Help but not yet used for filtering — ready for future extension.