[Certification Admin - Backend] Implement Endpoints to Define Coin Amount per Certificate

[Himindu-Kularathne]

Create backend endpoints that allow admins to configure the number of coins awarded for each certificate type.

• Endpoint to set/update coin amount for a specific certificate type (e.g., by certificate ID or name).
• Endpoint to retrieve coin amount for a given certificate.
• Persist data in a database (e.g., certificate ID to coin amount mapping).
• Add basic validations (e.g., non-negative coin values).
• Secure endpoints with admin authentication.