Remove DelayedJob worker and deliver mailers synchronously

[fwuensche]

Summary

• All async work in the app was 4 mailer deliveries on low-traffic paths (sign-in welcome, authorization request/grant/alert). Switch them to deliver_now, drop delayed_job_active_record, remove the worker dyno from Procfile, and drop the delayed_jobs table.
• Swap development to letter_opener so emails preview in the browser; remove perform_deliveries = false from test.rb so assert_emails can see deliveries (delivery_method = :test still prevents real SMTP).
• Updates affected tests to assert on ActionMailer::Base.deliveries instead of enqueued_jobs.

Post-merge action

Run `heroku ps:scale worker=0 --app cherrypush-production` to retire the worker formation (~$7/mo saved).

Test plan

• `bin/ci` passes (one pre-existing flaky system test unrelated to this change)
• Verify a sign-in in dev opens a welcome email in the browser via letter_opener
• After deploy, scale worker to 0 and confirm emails still deliver

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