Xchain Source Tx Recorded

Called by the frontend immediately after MetaMask returns a tx hash for the swapAndBridge call. Records the source_tx_hash on the bridge-attempt row, advancing its state from pending_signature to pending_bridge.

Body: { attempt_id: int, source_tx_hash: str }

Idempotent: re-POSTing with the same source_tx_hash for the same attempt is a no-op. Mismatched source_tx_hash on a row that already has one is rejected with 400 — exactly one source tx per attempt.

Per Comet review 2026-05-04 — without this, AlgoVoi can't reconcile Allbridge's destination tx back to the shopper's checkout.