Implement M2M Authentication for Internal Services

Add machine-to-machine authentication for internal service-to-service communication.

• Configure FusionAuth for M2M client credentials flow
• Services authenticate using client ID and secret
• Issue JWT tokens with appropriate scopes for service access
• Services needing M2M auth:
• Portal API
• Event Manager
• Other internal services as needed
• Implement token validation in receiving services
• Secure storage of client secrets (environment variables or secrets manager)

Open questions:

• What scopes/permissions should each service have?
• How will client secrets be rotated?
• Should services use short-lived tokens with refresh, or longer-lived tokens?