Add testing utilities and improve test coverage with FactoryBot integration

- Added `shoulda-matchers` for enhanced RSpec testing capabilities.
- Introduced `factory_bot_rails` for easier test data creation.
- Created factories for `HttpLogEntry`, `BlobEntry`, and `Domain::Fa::Post` models.
- Updated `rails_helper.rb` to include FactoryBot methods and configure Shoulda matchers.
- Enhanced `HttpLogEntry` model with a new `response_size` method.
- Refactored `justfile` to include parallel test execution.
- Improved `Gemfile` and `Gemfile.lock` with new testing gems.

Gemfile

@@ -87,6 +87,9 @@ group :test do
   gem "rails-controller-testing"
   gem "selenium-webdriver"
   gem "webdrivers"
+  gem "shoulda-matchers"
+  gem "factory_bot_rails"
+  gem "parallel_tests"
 end
 
 gem "xdiff", path: "/gems/xdiff-rb"