Pricing models API and management page

The pricing models page now loads live data from a new dedicated API endpoint, replacing the previous mock state.

The pricing models section is now fully wired to a persistent backend. Data is stored in a dedicated table and served through a new API route, so the page reflects real configuration instead of static placeholders.

Added a `GET /api/pricing-models` endpoint that reads and returns live pricing model records

Pricing models page replaced mock state with real API-backed data

Input validation added to the new endpoint to reject malformed requests early