Add model-level uniqueness validation on Subscription

[bellisabell]

Summary

Fixes #30 - Missing model-level uniqueness validation on Subscription

Changes

1. Model validation: Added validates :email_address_id, uniqueness: { scope: :account_id } to the Subscription model
2. Race condition handling: Added find_or_create_by_with_rescue class method that rescues ActiveRecord::RecordNotUnique and falls back to finding the existing record
3. Updated all creation sites to use the new helper:
   • PublicPagesController#create
   • SubscribersImportJob#import
   • Account#subscribe_to_updates

Why both?

• The model validation catches duplicates during normal operation and provides a friendly error message
• The RecordNotUnique rescue handles the race condition where two concurrent requests both pass the validation but one fails at the DB constraint level

Testing

• The existing DB index ensures data integrity
• The new validation will catch duplicates before hitting the database
• The rescue handler prevents 500 errors from race conditions