[Feature] Add Flutterwave payment gateway support alongside Paystack

[kelly-musk]

Problem

lib/bills/payment-gateway.ts references Flutterwave in comments and types but only Paystack is implemented. Flutterwave has broader coverage for payments across Kenya, Ghana, South Africa, Uganda, and Tanzania.

For Aframp's 12-country target market, Flutterwave is essential.

Fix

1. Implement FlutterwaveGateway class conforming to the PaymentGatewayService interface
2. Add Flutterwave webhook verification (HMAC with FLW-SECRET-HASH header)
3. Add a PAYMENT_GATEWAY env var to select the active gateway per country
4. Wire mobile money routes through Flutterwave's collection API

Impact

High product — Paystack-only support limits the platform to Nigerian users.

[Danitello123]

@Danitello123 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Please kindly assign me
God bless you

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[1sraeliteX]

@1sraeliteX has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello maintainer, I would like to work on this issue.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @1sraeliteX to this issue.