feat: apply affiliate rebate to subscription payments

[cugxuan]

订阅付款的时候仍然走返利

Summary

• apply affiliate rebate accrual to paid subscription order fulfillment, using the subscription order amount as the rebate base
• add a subscription assignment audit marker so fulfillment retries can skip duplicate subscription extension while still running rebate idempotency
• keep existing balance recharge rebate behavior unchanged and make rebate audit claim SQL work in unit-test SQLite as well as PostgreSQL

Tests

• go test -tags unit ./internal/service
• go test ./internal/service -run '^$'
• git diff --check

[github-actions]

All contributors have signed the CLA. ✅
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[cugxuan]

I have read the CLA Document and I hereby sign the CLA

[cugxuan]

issue #3370