CONV-245: Add subscription checkout route

[menvil]

Summary

• Added POST /billing/checkout/{plan} route → StartSubscriptionCheckoutController
• Controller validates plan key (404 for unknown, 422 for free), delegates to BillingPaymentService, redirects to checkout URL
• Added billing.success and billing.cancel named routes with minimal views (CONV-246 pre-included, needed for route() resolution in controller)
• No direct Cashier calls in controller — all goes through BillingPaymentService

Test plan

• Guest redirected to /login
• Auth user redirected to checkout URL (via fake gateway)
• Unknown plan returns 404
• Free plan returns 422
• composer test passes (441 tests)
• composer lint passes

🤖 Generated with Claude Code

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Summary by cubic

Adds subscription checkout flow with a new POST /billing/checkout/{plan} route that starts checkout and redirects to the payment gateway. Implements CONV-245 and wires the required success/cancel callbacks.

• New Features
  • Added POST /billing/checkout/{plan} (named billing.checkout), auth-only; redirects to the gateway checkout URL.
  • Controller validates plan (404 unknown, 422 free), delegates to BillingPaymentService, and uses billing.success/billing.cancel routes for return URLs.
  • Added minimal billing.success and billing.cancel views/routes; tests cover auth, redirect via fake gateway, and unknown/free plan errors.

^{Written for commit fee63bf. Summary will update on new commits.}

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• Commit unit tests in branch feature/CONV-245-add-subscription-checkout-route

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