[Feature]: Modular payment-processor plugin system (Grantfox & Stellar) #141

Feature Request

Problem Statement

Currently the codebase mixes payment logic, making it difficult to add new payment processors or maintain multiple backends (Grantfox, Stellar, others). Teams need a modular plugin system so adding or swapping processors is low-effort and reduces risk.

Proposed Solution

Design and implement a Payment Processor Plugin System:

• Define a clear TypeScript interface (IPaymentProcessor) with methods: initialize, createPayment, signTransaction, submitTransaction, getStatus, refund.
• Implement adapter plugins for Grantfox and Stellar that adhere to the interface.
• Provide runtime plugin registration, configuration via env or DB, and a factory to select the processor per request or tenant.
• Add end-to-end tests ensuring two processors can run side-by-side without interference.

Acceptance criteria:

• Clear TypeScript interface with documentation and tests for contract compliance.
• Two adapters implemented: GrantfoxAdapter and StellarAdapter, used by the PaymentsController.
• Dynamic selection of processor via config or request header works and is covered by tests.
• README documenting how to add new plugins and configuration examples.

Alternatives Considered

• Hardcoding payment flows per provider (rejected: poor maintainability).
• Monolithic payment service with feature flags (less flexible than plugin system).

Additional Context

A plugin system will make this repo more attractive for grant programs and external teams (Grantfox, Stellar) because it lowers onboarding friction and enables rewards for developing adapters.

Implementation Ideas (Optional)

• Use TypeScript generics for typed request/response shapes per processor.
• Provide a lightweight registry persisted in DB so operators can enable/disable processors at runtime.

Would you be willing to contribute?

• Yes, I'd like to submit a PR for this feature
• I can help with testing
• I can help with documentation
• I'd rather let someone else work on this