Support multiple allowed users for write access

MAPTOOL_ALLOWED_GITEA_USER now accepts a comma-separated list of
Gitea usernames instead of just one, for when more than one person
needs upload/delete/move access.

docs/architecture.adoc

@@ -260,9 +260,11 @@ username/password — there's already an account to reuse, and it keeps
 credential storage (password hash, resets, ...) out of this app
 entirely. Successfully authenticating with Gitea is not by itself
 enough to write, though — the callback also checks the returned
-`login` against `MAPTOOL_ALLOWED_GITEA_USER`, so registering (or
-already having) any other account on that Gitea instance doesn't grant
-access here.
+`login` against `MAPTOOL_ALLOWED_GITEA_USER` (a comma-separated
+allowlist, e.g. `randy,friend1,friend2`), so registering (or already
+having) any other account on that Gitea instance doesn't grant access
+here. It's a flat allowlist — everyone in it has equal write access,
+no per-user roles or permissions.
 
 Session state is a signed cookie (`Rack::Session::Cookie`, `secure`
 in production, `httponly`, `same_site: lax`) keyed off