feat: implement Fincra gateway with checkout, verification, refunds, and signed webhooks

[teesofttech]

Why

The README already mentions Fincra, but no enum, configuration, adapter, or tests exist. Fincra documents checkout by card, bank transfer, PayAttitude, and mobile money, with separate sandbox/live endpoints.

Official docs:

• https://docs.fincra.com/di/docs/checkout-1
• https://docs.fincra.com/reference/deposits-management-api

Scope

• Confirm the current API version, auth scheme, webhook signature, supported countries/currencies, and refund endpoint.
• Add config, enum/capability metadata, registration, create/verify/refund, and webhook verification.
• Use hosted redirect checkout to fit the current contract.
• Map amount/status/error semantics precisely.

Acceptance criteria

• Sanitized unit fixtures cover success, pending, decline, auth failure, rate limit, malformed response, and webhook signature.
• Sandbox create and verify pass in CI.
• Refund behavior is tested or marked unsupported in capability metadata.
• README/docs claim Fincra only after the implementation and tests merge.
• No credential is committed.