fix: align Stellar asset whitelist with verified assets

[pntech20]

Summary

• build backend supported assets from the verified Stellar asset registry
• preserve canonical asset casing for entries like yXLM while still accepting mixed-case input
• trim issuer input and cover verified assets, missing issuers, and issuer mismatches in config tests

Checks

• npm test -- --runInBand src/config/stellar.config.unit.spec.ts
• npx eslint src/config/stellar.config.ts src/config/stellar.config.unit.spec.ts
• git diff --check

Closes #437