Adds a /api/staff/file proxy that re-streams Civi attachments with
Content-Disposition: inline so a native <dialog> lightbox can preview
images and PDFs in place. Office docs keep their plain download link
and gain a "View in Google Docs" secondary link (uses the Civi-signed
URL so Google can fetch without our staff key).
Also threads mime through /api/staff/report (Attachment.get mime_type)
so the dispatcher picks the right affordance without relying solely on
filename inference.
