CONV-255/256: Test and implement monthly subscription credit grant

[menvil]

Summary

• CONV-255: Added failing tests for handleInvoicePaid — idempotency, free plan skip, metadata storage
• CONV-256: Implemented monthly credit grant via CreditLedger.grant() with idempotency
  • Free plan invoices are silently ignored
  • Duplicate invoices detected via json_extract(metadata_json, '$.stripe_invoice_id') query
  • Credit transaction reason = subscription_monthly_grant
  • Metadata includes stripe_invoice_id, plan, and raw payload
  • No direct balance mutation — all through CreditLedger

Test plan

• Duplicate invoice grants credits only once (1 transaction created)
• Free plan invoice creates no subscription_monthly_grant transaction
• Invoice ID and plan stored in credit transaction metadata
• composer test passes (459 tests)
• composer lint passes
• npm run build passes

🤖 Generated with Claude Code

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Summary by cubic

Implements monthly credit grants when a paid subscription invoice is processed, with idempotency. Adds tests to ensure a single grant, skipping free plans, and metadata storage (CONV-255, CONV-256).

• New Features
  • Paid plans only; free plan invoices are ignored.
  • One-time grant enforced by checking CreditTransaction metadata for stripe_invoice_id.
  • Uses CreditLedger->grant() with reason: subscription_monthly_grant; stores stripe_invoice_id, plan, and raw payload.

^{Written for commit 9c180e8. Summary will update on new commits.}

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• Commit unit tests in branch feature/CONV-255-test-paid-invoice-grants-monthly-credits-once

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