Group unassigned source records by site signal, largest groups first

Groups the organization's live source records that carry neither a site nor an entity by the normalized supply point, meter number, account number and location text they carry (a record appears once per (kind, value), however many fields carry it). Each group shows the active site its value already maps to, if any, and conflictingCount (see SiteSuggestion). Signal kinds rank supply_point > meter_number > account_number > location_text. The read runs two statements (the count, then the grouping), each under the server's statement timeout, so one call can take up to twice that timeout before answering 503. Error codes: 400 VALIDATION_ERROR (limit outside 1-200); 503 SUGGESTIONS_TIMEOUT.