Finalize Contact Import

Finalize an imported contact batch and trigger guarded auto-enrichment.

Trust boundary: LinkedIn/Google browser import modals currently insert contacts directly through Supabase, so the backend cannot write a receipt in the same commit. For a receipt not already written by a server import path, this endpoint creates it only after proving every supplied contact belongs to the authenticated organization and has created_at within the last 24 hours. That recency window limits which direct-client writes may be claimed as an import; it is not proof that the browser performed the original insert.