User Story:
As a developer, I want a service that verifies Stellar transactions so that only valid on-chain donations are recorded.
Acceptance Criteria:
- Fetches transaction from Stellar Horizon by txHash
- Verifies: destination matches campaign contract, operation type = payment, asset matches accepted assets
- Throws if transaction not found or not yet confirmed
- Retries up to 3 times with exponential backoff
User Story:
As a developer, I want a service that verifies Stellar transactions so that only valid on-chain donations are recorded.
Acceptance Criteria: