Feat(feature) Stellar micro-payments module

[akargi]

Description

Adds Stellar-specific convenience payment routes on top of the existing payment processor plugin system.

Changes

• Added POST /payments/stellar/create
• Added POST /payments/stellar/submit
• Added GET /payments/stellar/status
• Added server-side Stellar sign-and-submit support
• Added Stellar submit DTO and shared processor selector
• Ensured Stellar routes take precedence over generic payment routes
• Added tests and documentation

Closes #143

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Performance improvement
• Code refactoring

How Has This Been Tested?

• Unit tests added/updated
• E2E tests added/updated
• Manual testing performed
• Tested in staging environment

Validation performed:

npx jest --runInBand src/payments
npm run build:tsc

Results:

• 7 test suites passed
• 57 tests passed
• TypeScript build passed
• Tests use mocked payment and Stellar network clients; no network calls are made

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code where needed
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove the feature works
• New and existing relevant tests pass locally
• Any dependent changes have been merged and published in downstream modules

Screenshots

Not applicable. This change adds backend API routes only.

Additional Notes

The Stellar convenience routes explicitly select the Stellar processor and ignore PAYMENTS_DEFAULT_PROCESSOR.