feat: Implement P12 certificate generation improvements

- Replace hardcoded P12 password with randomly generated one
- Add p12_password column to certificates table
- Update certificate serializer to include p12 password in response
- Remove deprecated certificate revocation logic
- Add tests for certificate revocation functionality
- Implement async P12 generation via Sidekiq job
- Add job uniqueness to prevent parallel certificate generation

Migration changes:
- Replace p12_password_digest with p12_password column
- Add safety measures for column removal

db/migrate/20250319104749_change_p12_password_digest_to_p12_password_in_certificates.rb

@@ -0,0 +1,21 @@
+class ChangeP12PasswordDigestToP12PasswordInCertificates < ActiveRecord::Migration[6.1]
+  def up
+    add_column :certificates, :p12_password, :string
+
+    Certificate.find_each do |cert|
+      cert.update_column(:p12_password, cert.p12_password_digest)
+    end
+
+    safety_assured { remove_column :certificates, :p12_password_digest }
+  end
+
+  def down
+    add_column :certificates, :p12_password_digest, :string
+
+    Certificate.find_each do |cert|
+      cert.update_column(:p12_password_digest, cert.p12_password)
+    end
+
+    safety_assured { remove_column :certificates, :p12_password }
+  end
+end