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

test/jobs/p12_generator_job_test.rb

@@ -0,0 +1,25 @@
+require 'test_helper'
+
+class P12GeneratorJobTest < ActiveJob::TestCase
+  test "ensures only one job runs at a time" do
+    Sidekiq::Testing.inline!
+
+    api_user = users(:api_bestnames)
+    
+    thread1 = Thread.new do
+      P12GeneratorJob.perform_later(api_user.id)
+    end
+    
+    sleep(2)
+
+    thread2 = Thread.new do
+      P12GeneratorJob.perform_later(api_user.id)
+    end
+
+    thread1.join
+    thread2.join
+    
+  ensure
+    Sidekiq::Testing.fake!
+  end
+end