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/check_duplicates.php

@@ -5,22 +5,9 @@
  * Searches for tickets with similar titles using LIKE and SOUNDEX
  */
 
-// Apply rate limiting
-require_once dirname(__DIR__) . '/middleware/RateLimitMiddleware.php';
-RateLimitMiddleware::apply('api');
-
-session_start();
-require_once dirname(__DIR__) . '/config/config.php';
-require_once dirname(__DIR__) . '/helpers/Database.php';
+require_once __DIR__ . '/bootstrap.php';
 require_once dirname(__DIR__) . '/helpers/ResponseHelper.php';
 
-header('Content-Type: application/json');
-
-// Check authentication
-if (!isset($_SESSION['user']) || !isset($_SESSION['user']['user_id'])) {
-    ResponseHelper::unauthorized();
-}
-
 // Only accept GET requests
 if ($_SERVER['REQUEST_METHOD'] !== 'GET') {
     ResponseHelper::error('Method not allowed', 405);
@@ -33,15 +20,12 @@ if (strlen($title) < 5) {
     ResponseHelper::success(['duplicates' => []]);
 }
 
-// Use centralized database connection
-$conn = Database::getConnection();
-
 // Search for similar titles
 // Use both LIKE for substring matching and SOUNDEX for phonetic matching
 $duplicates = [];
 
 // Prepare search term for LIKE
-$searchTerm = '%' . $conn->real_escape_string($title) . '%';
+$searchTerm = '%' . $title . '%';
 
 // Get SOUNDEX of title
 $soundexTitle = soundex($title);