chore: remove EVM ethers.js dependency

- Removed ethers.js v6.13.5 (EVM library)
- Repository now 100% Stellar/Soroban compliant
- Uses @stellar/stellar-sdk for blockchain operations

EVM Audit: Removed all EVM dependencies. Backend now clean.

Author: elizabetheonoja-art

package.json

@@ -74,7 +74,6 @@
     "dataloader": "^2.2.2",
     "date-fns": "^4.1.0",
     "dotenv": "^17.4.2",
-    "ethers": "^6.13.5",
     "express": "^5.2.1",
     "express-rate-limit": "^8.5.1",
     "fluent-ffmpeg": "^2.1.2",