Add instructor email notifications for pending requests

[alxstx]

Changes

• Added pending_notification_frequency and pending_notification_email columns to course_settings
• Added validations, normalization callbacks, and query scope to CourseSettings model
• Added PendingRequestsNotificationJob to send digest emails with pending request count and link
• Added pending notification params to CourseSettingsController permit list
• Added rake task notifications:send_pending_digests[daily|weekly] for cron scheduling
• Added frequency dropdown and notification email field to General Settings tab
• Added Stimulus toggle to disable email field when no frequency is selected

Testing

• Run bundle exec rspec spec/models/course_settings_spec.rb for model validation and scope tests
• Run bundle exec rspec spec/jobs/pending_requests_notification_job_spec.rb for job tests
• Run bundle exec rspec spec/controllers/course_settings_controller_spec.rb for controller tests
• Run bundle exec rspec spec/tasks/notifications_rake_spec.rb for rake task tests
• Run the full rspec test suite to verify no regressions

Documentation

No documentation needed

Checklist

• Name of branch corresponds to story
• 80%+ test coverage with all tests passing

Closes #344