Attribute Mappings Row Counts

Lazy per-mapping contact_attributes row counts for the page.

Split out of GET /attribute-mappings, which used to count inline: counting a mapping's owned rows is a full table scan for large orgs (Pingo: ~60M rows) and blocked every list render. The page loads the mappings first, then calls this with the loaded ids to fill the "Rows" column. Read-only POST (the id list is the request body) — no audit log, no side effects. Declared above /{mapping_id} so the static path isn't shadowed by the id route.