Stamp every unassigned record carrying a signal with a site (idempotent)

Stamps every live source record of the organization that carries the signal (the value is normalized like a matcher value) and has neither a site nor an entity, and its emission rows, with the site, in batches, each audited with reason assign. A record is skipped (and counted in skippedConflictingCount) only when its best matched site (that of its highest-ranked signal with an active site's matcher) comes from a signal kind ranked above the assigned kind and is not the target site; a site matched only by an equal or lower-ranked signal does not block it. Kinds rank supply_point > meter_number > account_number > location_text. Each batch commits on its own: if the call fails part-way, records already stamped stay stamped, and re-running the same call stamps the rest (a full re-run stamps nothing new). Each batch's statements (and the final skipped count) run under the server's statement timeout; a batch that exceeds it rolls back alone and the call answers 503 ASSIGN_TIMEOUT. The batches before it stay stamped, so re-running the same call is safe and continues. A bulk back-fill is a change an admin confirms, so the call always needs access.manage (checked before the body is validated); a single record's correction is POST /emissions/site-assignment, which needs org.write_data. createMatcher also creates the matcher (or reuses the one this value already has for the same site). Error codes: 400 VALIDATION_ERROR; 403 FORBIDDEN (no access.manage); 409 MATCHER_EXISTS (the value maps to another site); 422 SITE_NOT_ASSIGNABLE (unknown, archived or another organization's site); 503 ASSIGN_TIMEOUT (re-run the same call to continue).