admin: user management, invites, password reset, audit log

Add /admin/users to create, edit, disable and delete accounts with the
per-area permission grid. New users get a one-time invite link (48 h)
by email; the admin also sees it once, so an account without email can
still be onboarded. Forgotten passwords get a 1 h reset link with the
same answer for unknown accounts. Only token hashes are stored.

Guards against escalation and lockout: a user manager can only grant
flags they hold and cannot touch flags they lack; only superusers change
superuser accounts; nobody can disable or delete themselves; the last
active superuser stays.

Logins, user changes, and bingo and invoice actions are recorded in an
audit log at /admin/audit.

Read timestamps as UTC: they are all written with Time.now.utc, and on a
UTC+4 host a fresh 1 h token read back as already expired.

Assisted-by: Claude:opus-5

lib/audit.rb

@@ -0,0 +1,20 @@
+require 'sequel'
+
+# Append-only record of who changed what in the admin (/admin/audit).
+module Audit
+  module_function
+
+  def log(action:, user: nil, target: nil, detail: nil, ip: nil, username: nil)
+    DB.connection[:audit_log].insert(
+      user_id: user&.id, username: user&.username || username, action: action,
+      target: target&.to_s&.slice(0, 200), detail: detail&.to_s&.slice(0, 2000),
+      ip: ip, created_at: Time.now.utc
+    )
+  rescue Sequel::Error => e
+    $logger.error("audit: #{action} not recorded: #{e.message}") # never break the action itself
+  end
+
+  def recent(limit = 200)
+    DB.connection[:audit_log].order(Sequel.desc(:created_at), Sequel.desc(:id)).limit(limit).all
+  end
+end