Periodically re-sync session privileges from Authelia (#56)

AuthMiddleware::authenticate() only re-read Remote-User/Remote-Groups
(and thus is_admin, via UserModel::syncUserFromAuthelia) when
$_SESSION['user'] didn't exist yet. Once a session existed, every
subsequent request only checked the idle timer — never re-validating
against current Authelia/LLDAP state. An admin's group membership
revoked in LLDAP, or a logout at the Authelia proxy, left their
already-open session with full access for up to SESSION_TIMEOUT (5h
default), with no way to force early revocation short of clearing the
server-side session store.

Added PRIVILEGE_RESYNC_INTERVAL (default 5 min, matching
UserModel's own cache TTL) and a resyncPrivileges() check on every
already-authenticated request past that interval: re-reads the current
request's forward-auth headers (enforcing the trusted-proxy check
again, same as a fresh login), and either destroys the session and
redirects to re-auth if the user no longer has any required group, or
re-syncs is_admin/groups/display_name/email if they do. Best-effort if
this particular request doesn't carry forward-auth headers at all
(skips silently rather than force-logging out, retried next interval).

UserModel::syncUserFromAuthelia() has its own 5-minute in-process
cache keyed only by username (not by the groups being synced), so a
naive re-call during a resync would have kept returning the
pre-revocation cached result for up to 5 more minutes — invalidated
that cache entry immediately beforehand to guarantee a real re-sync.

Verified against real MariaDB across a fresh login, a same-interval
request confirming no premature resync, an admin-privilege-revocation
mid-session (is_admin flips to false in both session and DB, verified
via a direct query), and a full group-membership revocation (session
destroyed, redirected to re-auth, confirmed the request never reaches
past that point).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Lhz7pGMaoTfL5sdYS5XiKv

config/config.php

@@ -106,6 +106,11 @@ $GLOBALS['config'] = [
     'SESSION_TIMEOUT' => 18000, // 5 hours in seconds
     'SESSION_REGENERATE_INTERVAL' => 300, // Regenerate session ID every 5 minutes
 
+    // How often an already-logged-in session re-validates Remote-User/
+    // Remote-Groups against current Authelia/LLDAP state (AuthMiddleware).
+    // Without this, a revoked admin keeps full access for up to SESSION_TIMEOUT.
+    'PRIVILEGE_RESYNC_INTERVAL' => 300, // 5 minutes
+
     // CSRF settings
     'CSRF_LIFETIME' => 3600, // 1 hour in seconds