Bulk update sweep threshold configurations

Upserts multiple sweep thresholds in a single transaction: a threshold row is created on first configuration of an asset and updated thereafter. Each (tickerId, ledgerId) pair must exist in the Custody catalog. All updates are applied atomically (all-or-nothing); if any update fails, the entire request is rolled back. Optimistic locking: send the version from the corresponding GET row in each update. If any submitted version no longer matches the stored row (or a version is sent for an asset that has since been configured), the whole request is rejected with 409 and nothing is written.