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>

views/CreateTicketView.php

@@ -13,6 +13,7 @@ $nonce = SecurityHeadersMiddleware::getNonce();
     <link rel="icon" type="image/png" href="<?php echo $GLOBALS['config']['ASSETS_URL']; ?>/images/favicon.png">
     <link rel="stylesheet" href="<?php echo $GLOBALS['config']['ASSETS_URL']; ?>/css/dashboard.css?v=20260126c">
     <link rel="stylesheet" href="<?php echo $GLOBALS['config']['ASSETS_URL']; ?>/css/ticket.css?v=20260124e">
+    <script nonce="<?php echo $nonce; ?>" src="<?php echo $GLOBALS['config']['ASSETS_URL']; ?>/js/utils.js"></script>
     <script nonce="<?php echo $nonce; ?>" src="<?php echo $GLOBALS['config']['ASSETS_URL']; ?>/js/dashboard.js?v=20260205"></script>
     <script nonce="<?php echo $nonce; ?>">
     // CSRF Token for AJAX requests
@@ -316,12 +317,6 @@ $nonce = SecurityHeadersMiddleware::getNonce();
             });
     }
 
-    function escapeHtml(text) {
-        const div = document.createElement('div');
-        div.textContent = text;
-        return div.innerHTML;
-    }
-
     function toggleVisibilityGroups() {
         const visibility = document.getElementById('visibility').value;
         const groupsContainer = document.getElementById('visibilityGroupsContainer');