Get a payable obligation by ID

Verified staff/service with payments:read or staff with payments:operations:write authority is required. Read exact registered source and current reserved, paid, returned, unallocated and unpaid amounts, associated remittance IDs and explicit readiness blockers. Customer authority is refused. Requests are strict single JSON objects bounded to 8 MiB; complete responses are bounded to 16 MiB and never silently truncated.