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