stellar.service.ts: retry logic causes duplicate transactions — double payout

[nonsobethel0-dev]

stellar.service.ts: retry logic reuses stale transaction object — sequence number may be out of sync

invokeContract retries with MAX_ATTEMPTS=3 and re-fetches the account to get a fresh sequence number. However, if the first attempt partially succeeds (tx submitted but response lost), the retry builds a new tx with a different sequence, and both land on the blockchain.

Acceptance criteria:

• After each failed attempt, check if the tx actually landed on chain before retrying
• Use a unique client-side ID (timestamp + random) to detect duplicates
• If a duplicate is detected, return the original txHash instead of submitting again