feat: improve certificate download extensions

Update certificate download functionality to use appropriate file extensions:
- Use .p12 extension for PKCS#12 files
- Keep .pem extension for PEM-encoded files (CSR, CRT, private key)

This change ensures that downloaded certificate files have the correct extension based on their format, making it easier for users to identify and use the files correctly.

app/models/api_user.rb

@@ -20,7 +20,6 @@ class ApiUser < User
   # TODO: should have max request limit per day?
   belongs_to :registrar
   has_many :certificates
-  has_many :user_certificates
 
   validates :username, :plain_text_password, :registrar, :roles, presence: true
   validates :plain_text_password, length: { minimum: min_password_length }