CONV-269: Implement credit pack purchase idempotency

[menvil]

Summary

• Wraps grant logic in DB::transaction() with a checkout session ID check
• alreadyGrantedForCheckoutSession() queries credit_transactions by metadata_json->stripe_checkout_session_id
• Same checkout session ID (even with different event IDs) does not grant credits twice
• No separate table created — metadata query is sufficient

Test plan

• Duplicate same event → credits granted once
• Same checkout session, different event IDs → credits granted once
• composer test passes (481 tests)
• composer lint passes

🤖 Generated with Claude Code

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Summary by cubic

Implements credit pack checkout with Stripe and makes credit granting idempotent using the Checkout Session ID so duplicate or replayed events don’t grant credits twice. Satisfies CONV-269.

• New Features

  • Config-driven credit packs (config/billing.php) with simple checkout: POST /billing/credits/{pack} redirects to Stripe; success/cancel views added.
  • Idempotent webhook handling: requires Checkout Session ID, wraps grant in DB::transaction(), and skips if credit_transactions.metadata_json->stripe_checkout_session_id already exists.
  • Stripe integration via CashierCreditPackCheckoutGateway; metadata includes user_id, pack_key, pack_credits; tests cover flow and idempotency.

• Migration

  • Set STRIPE_CREDIT_PACK_SMALL_PRICE_ID, STRIPE_CREDIT_PACK_MEDIUM_PRICE_ID, STRIPE_CREDIT_PACK_LARGE_PRICE_ID.
  • Point Stripe checkout.session.completed webhooks to POST /stripe/webhook.
  • No database changes required.

^{Written for commit 9031d7f. Summary will update on new commits.}

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feature/CONV-269-implement-credit-pack-purchase-idempotency

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