backend: harden asset issuer handling

[edehvictor]

Closes #607
Closes #608
Closes #606
Closes #609

Changes

• Resolve configured default issuers for standard non-native assets like USDC before allowlist checks and inserts.
• Validate asset_issuer values as Stellar public keys in schema, route, service, and Stellar asset resolution paths.
• Keep native XLM issuerless so issuer allowlists only apply to non-native assets.
• Update focused schema/service tests and route test mocks for the new issuer validation helper.

Testing

• git diff --check
• node --check backend/src/lib/request-schemas.js
• node --check backend/src/services/paymentService.js
• node --check backend/src/routes/payments.js
• node --check backend/src/lib/stellar.js
• node --check backend/src/constants/assetConstants.js
• node --check backend/src/lib/request-schemas.test.js
• node --check backend/src/services/paymentService.test.js

Full test suite not run; dependencies were not installed per request.

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