[FEATURE] Soroban swap contract: crypto trades happen off-chain with no on-chain proof

[derbydaniels9-cpu]

Summary

There is no Soroban contract for the crypto swap functionality. Currently, all XLM/USDC buy and sell operations happen off-chain — the backend simply updates database balance fields. This means swaps are not verifiable on-chain and users must trust the backend completely.

Proposed contract

A Soroban swap contract that:

1. Accepts XLM or USDC from the user's Stellar address.
2. Executes a real on-chain swap via Stellar's built-in DEX (order book or liquidity pool path).
3. Credits the output asset directly to the user's wallet.
4. Emits a SwapExecuted event the backend can index.

Benefits

• Swaps are verifiable on Stellar explorer — full transparency.
• Eliminates the backend as a trusted intermediary for the swap rate.
• Users get the live market rate from Stellar DEX instead of the backend's hardcoded NGN_USD_RATE.

Stellar DEX path payment reference

// Soroban can invoke Stellar's path payment operation
Operation::path_payment_strict_send(
    send_asset: Asset::native(),   // XLM
    send_amount: amount,
    destination: user_address,
    dest_asset: USDC_asset,
    dest_min: min_receive,
    path: vec![],
)

References

• Stellar DEX docs
• Soroban path payment