[CT-35] Implement StellarClient::anchor_transfer() — currently a no-op that fabricates anchored records

[mftee]

Overview

pub async fn anchor_transfer(&self, _transfer_hash: &str, _memo: &str) -> Result<()> {
    Ok(())
}

Both parameters are prefixed with underscore because they're unused, and the function does nothing but return success. record_transfer() in lib.rs calls this and, on its Ok(()) result, proceeds to persist a TransferRecord as if the transfer had actually been anchored on Stellar. Right now every call to POST /transfer fabricates an anchored record without ever submitting anything to the Stellar network.

File

contract/src/stellar.rs (anchor_transfer)

Acceptance Criteria

• Implement anchor_transfer to actually submit a Stellar transaction (a manageData or payment-with-memo operation carrying the transfer_hash/memo), following the same signing pattern the NestJS StellarService.anchorHash uses
• Requires a configured signing keypair — wire config.stellar_secret_key (already parsed/validated in config.rs) into StellarClient's construction
• Return a real error (not swallowed) when the Stellar submission fails, so record_transfer's existing error-handling path (which already logs and returns 500) actually triggers on genuine failures
• Add a test verifying anchor_transfer submits a transaction (mocked Horizon) rather than trivially returning Ok