PKG server: email templates and demo API keys for Bereken je Recht

[rubenhensen]

Parent issue

#81

Summary

Add support for email templates linked to API keys and a demo mode with rate limiting on the PKG server.

Requirements

1. Add an email template for Bereken je Recht (PKG server / SQL database)
2. Add a demo boolean in the API key store with aggressive rate limiting implemented
3. Support adding an email template to an API key

Deliverables

• Database schema changes for email templates and demo flag
• API endpoints or admin tooling to manage email templates per API key
• Rate limiting logic for demo API keys