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

views/admin/login.erb

@@ -12,4 +12,5 @@
         <div class="field"><label for="password">Password</label><input type="password" id="password" name="password" autocomplete="current-password" required /></div>
         <button type="submit" class="download-button">Log in</button>
     </form>
+    <p class="optional" style="text-align: center"><a href="/admin/reset">Forgot password?</a></p>
 </section>