[CT-21] Build SignatureVerificationService

[mftee]

Problem

There is no way to verify that a Stellar transaction recorded in the system was actually signed by a specific keypair. An attacker who knows a transaction hash could claim authorship without having signed it.

Proposed Solution

Create a SignatureVerificationService inside contract/module/signature/ that queries Stellar Horizon to verify transaction signers.

Acceptance Criteria

• verify_signature(tx_hash: &str, expected_public_key: &str) async function queries the Stellar Horizon transaction endpoint for the given tx_hash
• Returns Ok(true) if the expected_public_key appears in the transaction's signers list
• Returns Ok(false) if the transaction exists but the public key is not among its signers
• Returns Err if the transaction is not found on Stellar (404 from Horizon) or if Horizon is unreachable
• GET /module/signature/verify accepts tx_hash and public_key query parameters and returns { verified: bool, signers: [string] }
• Unit tests mock the Horizon HTTP response to cover: signer found, signer not found, transaction not found, and Horizon error
• Implementation files live inside contract/module/signature/