Add SQLite-backed announcement system and admin portal

- Add SQLite database initialization and management in inc/db.php
- Implement session authentication and CSRF security helpers in inc/auth.php
- Implement announcement querying and saving in inc/announcements.php
- Add management dashboard, login, and logout in admin/
- Protect db/ directory with Apache access controls

admin/logout.php

@@ -0,0 +1,5 @@
+<?php
+require_once __DIR__ . '/../inc/auth.php';
+logout_user();
+header('Location: login.php');
+exit;