fix: #3638001 Anonymous users cannot flag anymore when flood control is enabled

Julian Pustkuchenrequested to merge
issue/flag-3638001:3638001-5.2.x-regression-anonymous-users-flagging-flood into 5.2.x

Anonymous users were identified for flood control by IP address only, so all anonymous visitors behind a reverse proxy, container network or shared NAT shared one flood bucket, and the flag link disappeared for everyone.

  • Identify anonymous users by their flag session ID instead, reading the lazily started session so the first flagging is counted.
  • Add a separate, configurable per-IP limit for anonymous users (flood_control_ip_limit, default 50, 0 disables) so that discarding the session cookie does not circumvent flood control.
  • Keep the page cache from storing pages where an anonymous user is denied.
  • Add flag_update_10302() for the new setting.

Co-Authored-By: Claude Opus 5.5 noreply@anthropic.com

Related to #3638001

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