DB-gestützte Sessions (opt-in) + 'überall abmelden'

- includes/DbSessionHandler.php (SessionHandlerInterface, fehlertolerant)
- Auth: opt-in-Registrierung (Setting session_driver=db), activeSessionCount(),
  logoutOtherSessions()
- Migration 0004 (sessions.user_id NULL) + database.sql
- Settings-Toggle in integrations.php; 'überall abmelden' in security.php

database.sql

@@ -114,7 +114,7 @@ CREATE TABLE IF NOT EXISTS `vouchers` (
 
 CREATE TABLE IF NOT EXISTS `sessions` (
   `id` VARCHAR(128) PRIMARY KEY,
-  `user_id` INT NOT NULL,
+  `user_id` INT NULL,
   `data` TEXT,
   `expires_at` TIMESTAMP NOT NULL,
   `created_at` TIMESTAMP DEFAULT CURRENT_TIMESTAMP,