Seed an organization-wide Area catalogue (East, Central, West, California)
with stable ids, add a nullable AreaId to VendorCompany, allow only Admins
to change it through the roster endpoints, expose areas facet metadata and
an areas[n] company-directory filter with the __unassigned__ sentinel.
PATCH /api/vendor-company-roster/{companyId} inserts the submitted
technicians and optionally updates company fields. Technicians absent
from the payload are never removed or deactivated, so the Add Vendor
flow can no longer soft-delete an existing roster via the full-snapshot
PUT. Stale rowVersion still 409s; unknown company 404s. POST (create)
and PUT (reconcile) behaviour is unchanged.
