During account enrichment, WebsiteBrandingService now probes the signup
domain's MX records to infer whether email is hosted on Google Workspace
or Microsoft 365, adding `email_provider` (`google`/`microsoft`/`nil`)
to `brand_info`. Matching is anchored on a label boundary so lookalike
domains aren't misclassified, and lookup failures fall back to `nil`.
This lets downstream UI suggest the right mailbox integration (Gmail vs
Outlook).