Get available payment methods — API operation

[rammrain]

Summary

Implement the "get available payment methods" API call.

Requirements

• GET available payment methods, optionally filtered by country (EE, LV, LT, FI, PL, DE)
• Return structured response grouped by country
• Include bank transfers, card payments, BNPL, and hire purchase where available
• Each method exposes bank/provider code and display metadata (name, logo URL)
• Define response DTOs for payment method listings
• Expose via a service interface for testability

Refs

• initial-docs/project1.md — payment method discovery, grouped by country
• initial-docs/project2.md — available methods per country, provider codes, display metadata

Testing

Unit tests with stubbed HTTP layer: successful retrieval, multiple countries, empty results, various method types. Target near-perfect coverage.