documents.visible() now takes the viewer's unit set. A members document is
served and listed only to a signed-in member of the matching unit; 'both'
reaches any member; owner and admin reach everything including unit types
added later. Everyone else gets 404, never 403.
The gate moved INSIDE find(), so there is one path from a slug to a file and
no route can forget to check. listed() and visible() stay separate functions.
admin_api takes a session first and falls back to X-Admin-Token as break
glass. Still fails closed: no session and no ADMIN_TOKEN is 503. Capability,
not role, decides per route. announcements.created_by now comes from the
session and ignores any value in the request body.
tests/smoke_documents.py, 24 checks, including the invariant that the index
can never list something serving would refuse.
