admin: publish files to the public bucket

There was no way to put a file up for public use short of running an
S3 client with the uploader key. Add a "files" area to the admin:
upload one or more files, copy the link, edit the note, replace the
contents under the same link, delete. It has its own read/write/delete
permission flags like the other areas, and every action is audited.

The objects go to the storage stack's public bucket and are served by
Garage at s3.asxp.io/public/<key>, not by this app. The app's key can
only put and delete there, so a public_files table is the record of
what was published. Keys get a random prefix: links are not guessable
and names never collide.

That host is same-site with the admin, so a file that can carry script
must not render there. Only a fixed list of types (images, PDF, plain
text, audio, video) is served inline; the rest, HTML and SVG included,
is stored as an attachment. Image metadata is stripped by default, as
for the bingo photos, because of GPS tags.

A per-file limit and a total quota keep the small disk on asxpio-ops
from filling up.

Assisted-by: Claude:fable-5-2

views/admin/_nav.erb

@@ -1,5 +1,5 @@
 <nav class="admin-nav">
-    <% [['/admin', 'dashboard', nil], ['/admin/invoices', 'invoices', 'invoices'], ['/admin/bingo', 'bingo', 'bingo'], ['/admin/users', 'users', 'users']].each do |href, label, area| %>
+    <% [['/admin', 'dashboard', nil], ['/admin/invoices', 'invoices', 'invoices'], ['/admin/bingo', 'bingo', 'bingo'], ['/admin/files', 'files', 'files'], ['/admin/users', 'users', 'users']].each do |href, label, area| %>
         <% next if area && !can?(area, 'read') %>
         <a href="<%= href %>" class="<%= 'active' if href == '/admin' ? request.path == '/admin' : request.path.start_with?(href) %>"><%= label %></a>
     <% end %>