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

lib/user.rb

@@ -8,7 +8,7 @@ require 'uri'
 # Admin accounts. Passwords are scrypt hashes from Ruby's OpenSSL (no gem):
 # "scrypt$<logN>$<r>$<p>$<salt>$<hash>", base64 salt/hash.
 class User < Sequel::Model(:users)
-  AREAS    = %w[invoices bingo users].freeze
+  AREAS    = %w[invoices bingo files users].freeze
   PERMS    = %w[read write delete].freeze
   NAME_RE  = /\A[a-z0-9][a-z0-9_.@-]{1,63}\z/
   MIN_PASS = 12