Extract Stripe Domain Models and Simplify Webhook Handling

[frogr]

Problem

Stripe webhook handling is spread across multiple services with mixed responsibilities.

Tasks

• Create domain models for Stripe events (e.g., StripeEvent, CheckoutSession, SubscriptionEvent)
• Extract event handlers into separate command objects
• Implement event factory pattern
• Simplify StripeWebhookService to only orchestrate

Acceptance Criteria

• Create app/models/stripe/ directory with domain models
• Each webhook event type has its own command class
• StripeWebhookService reduced to <50 lines
• All existing webhook specs pass
• Add unit tests for new domain models
• Integration tests remain unchanged

Files to modify

• app/services/stripe_webhook_service.rb
• app/services/stripe_signature_verification_service.rb
• Create new files in app/models/stripe/
• Update spec/services/stripe_webhook_service_spec.rb

Phase: 2 (Domain)
Priority: High