Audit fixes: security, dead code removal, API consolidation, JS dedup

Security:
- Fix IDOR in delete/update comment (add ticket visibility check)
- XSS defense-in-depth in DashboardView active filters
- Replace innerHTML with DOM construction in toast.js
- Remove redundant real_escape_string in check_duplicates
- Add rate limiting to get_template, download_attachment, audit_log,
  saved_filters, user_preferences endpoints

Bug fixes:
- Session timeout now reads from config instead of hardcoded 18000
- TicketController uses $GLOBALS['config'] instead of duplicate .env parsing
- Add DISCORD_WEBHOOK_URL to centralized config
- Cleanup script uses hashmap for O(1) ticket ID lookups

Dead code removal (~100 lines):
- Remove dead getTicketComments() from TicketModel (wrong bind_param type)
- Remove dead getCategories()/getTypes() from DashboardController
- Remove ~80 lines dead Discord webhook code from update_ticket API

Consolidation:
- Create api/bootstrap.php for shared API setup (auth, CSRF, rate limit)
- Convert 6 API endpoints to use bootstrap
- Extract escapeHtml/getTicketIdFromUrl into shared utils.js
- Batch save for user preferences (1 request instead of 7)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

api/get_users.php

@@ -4,25 +4,9 @@
  * Returns list of users for @mentions autocomplete
  */
 
-ini_set('display_errors', 0);
-error_reporting(E_ALL);
-
-require_once dirname(__DIR__) . '/middleware/RateLimitMiddleware.php';
-RateLimitMiddleware::apply('api');
+require_once __DIR__ . '/bootstrap.php';
 
 try {
-    require_once dirname(__DIR__) . '/config/config.php';
-    require_once dirname(__DIR__) . '/helpers/Database.php';
-
-    // Check authentication (session already started by RateLimitMiddleware)
-    if (!isset($_SESSION['user']) || !isset($_SESSION['user']['user_id'])) {
-        http_response_code(401);
-        echo json_encode(['success' => false, 'error' => 'Authentication required']);
-        exit;
-    }
-
-    header('Content-Type: application/json');
-
     // Get all users for mentions/assignment
     $result = Database::query("SELECT user_id, username, display_name FROM users ORDER BY display_name, username");