Mark onramp event consumed after SA->tipping transfer submission

Mark a settled onramp event as consumed after the frontend submits the SA→tipping wallet transfer transaction.

This is the final step: the frontend sends the settled amount from the Smart Account to the tipping wallet, then calls this endpoint with the transaction hash.

Idempotent: If already consumed with the same tx_hash, returns current state.

Error codes:

• 400: Invalid transfer_tx_hash format (must be 0x + 64 hex chars)
• 404: Event not found or doesn't belong to authenticated account
• 409: Event not in confirmed status, or consumed with a different tx_hash