Create a new encrypted secret for the authenticated user's space.

This endpoint:

• Requires API key authentication
• Encrypts the secret value using AWS KMS
• Stores the encrypted value in the database
• Returns metadata about the created secret (including id for future reference)

Args: service: Injected secrets service payload: Request containing secret_name and secret_value

Returns: Response with created secret metadata including id

Raises: 400 Bad Request: If user doesn't have an associated space 500 Internal Server Error: If encryption or storage fails