Implement comprehensive improvement plan (Phases 1-6)

Security (Phase 1-2):
- Add SecurityHeadersMiddleware with CSP, X-Frame-Options, etc.
- Add RateLimitMiddleware for API rate limiting
- Add security event logging to AuditLogModel
- Add ResponseHelper for standardized API responses
- Update config.php with security constants

Database (Phase 3):
- Add migration 014 for additional indexes
- Add migration 015 for ticket dependencies
- Add migration 016 for ticket attachments
- Add migration 017 for recurring tickets
- Add migration 018 for custom fields

Features (Phase 4-5):
- Add ticket dependencies with DependencyModel and API
- Add duplicate detection with check_duplicates API
- Add file attachments with AttachmentModel and upload/download APIs
- Add @mentions with autocomplete and highlighting
- Add quick actions on dashboard rows

Collaboration (Phase 5):
- Add mention extraction in CommentModel
- Add mention autocomplete dropdown in ticket.js
- Add mention highlighting CSS styles

Admin & Export (Phase 6):
- Add StatsModel for dashboard widgets
- Add dashboard stats cards (open, critical, unassigned, etc.)
- Add CSV/JSON export via export_tickets API
- Add rich text editor toolbar in markdown.js
- Add RecurringTicketModel with cron job
- Add CustomFieldModel for per-category fields
- Add admin views: RecurringTickets, CustomFields, Workflow,
  Templates, AuditLog, UserActivity
- Add admin APIs: manage_workflows, manage_templates,
  manage_recurring, custom_fields, get_users
- Add admin routes in index.php

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

api/check_duplicates.php

@@ -0,0 +1,117 @@
+<?php
+/**
+ * Check for duplicate tickets API
+ *
+ * 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/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 ($_SERVER['REQUEST_METHOD'] !== 'GET') {
+    ResponseHelper::error('Method not allowed', 405);
+}
+
+// Get title parameter
+$title = isset($_GET['title']) ? trim($_GET['title']) : '';
+
+if (strlen($title) < 5) {
+    ResponseHelper::success(['duplicates' => []]);
+}
+
+// Create database connection
+$conn = new mysqli(
+    $GLOBALS['config']['DB_HOST'],
+    $GLOBALS['config']['DB_USER'],
+    $GLOBALS['config']['DB_PASS'],
+    $GLOBALS['config']['DB_NAME']
+);
+
+if ($conn->connect_error) {
+    ResponseHelper::serverError('Database connection failed');
+}
+
+// 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) . '%';
+
+// Get SOUNDEX of title
+$soundexTitle = soundex($title);
+
+// First, search for exact substring matches (case-insensitive)
+$sql = "SELECT ticket_id, title, status, priority, created_at
+        FROM tickets
+        WHERE (
+            title LIKE ?
+            OR SOUNDEX(title) = ?
+        )
+        AND status != 'Closed'
+        ORDER BY created_at DESC
+        LIMIT 10";
+
+$stmt = $conn->prepare($sql);
+$stmt->bind_param("ss", $searchTerm, $soundexTitle);
+$stmt->execute();
+$result = $stmt->get_result();
+
+while ($row = $result->fetch_assoc()) {
+    // Calculate similarity score
+    $similarity = 0;
+
+    // Check for exact substring match
+    if (stripos($row['title'], $title) !== false) {
+        $similarity = 90;
+    }
+    // Check SOUNDEX match
+    elseif (soundex($row['title']) === $soundexTitle) {
+        $similarity = 70;
+    }
+    // Check word overlap
+    else {
+        $titleWords = array_map('strtolower', preg_split('/\s+/', $title));
+        $rowWords = array_map('strtolower', preg_split('/\s+/', $row['title']));
+        $matchingWords = array_intersect($titleWords, $rowWords);
+        $similarity = (count($matchingWords) / max(count($titleWords), 1)) * 60;
+    }
+
+    if ($similarity >= 30) {
+        $duplicates[] = [
+            'ticket_id' => $row['ticket_id'],
+            'title' => $row['title'],
+            'status' => $row['status'],
+            'priority' => $row['priority'],
+            'created_at' => $row['created_at'],
+            'similarity' => round($similarity)
+        ];
+    }
+}
+
+$stmt->close();
+
+// Sort by similarity descending
+usort($duplicates, function($a, $b) {
+    return $b['similarity'] - $a['similarity'];
+});
+
+// Limit to top 5
+$duplicates = array_slice($duplicates, 0, 5);
+
+$conn->close();
+
+ResponseHelper::success(['duplicates' => $duplicates]);