feat: Stellar network configuration (testnet/mainnet)

[Elite-tch]

Description
This PR implements multi-network support for Stellar and Soroban, allowing the backend to switch between testnet and mainnet via environment variables.

Changes
Dynamic Config: Added
src/config.ts
to handle network detection via STELLAR_NETWORK or SOROBAN_NETWORK.
Client Initialization: Created
src/lib/stellar.ts
to initialize Horizon and Soroban RPC servers based on the active network.
Contract Mapping: Configured separate Vault and Settlement contract IDs per network.
API Updates: Updated health check endpoint to report current network status.
Documentation: Added
.env.example
and updated
README.md
with environment variable details.
Environment Variables
STELLAR_NETWORK: testnet (default) or mainnet.
Network-specific overrides for RPC URLs and Contract IDs (e.g., TESTNET_VAULT_CONTRACT_ID, MAINNET_RPC_URL).
Verification
Verified via GET /api/health that the service correctly switches URLs and network passphrases when the environment variable is toggled.

close issue #57

[greatest0fallt1me]

please resolve the conflicts

[Elite-tch]

done

[greatest0fallt1me]

@Elite-tch Can you resolve the conflicts?

[Elite-tch]

please i have resolve the issue . and please can you check my pr , i first created pr but you have been merging other peoples pr which have been causing conflict on my own , is actually unfair . please consider checking my pr before another conflict thanks