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>

helpers/ResponseHelper.php

@@ -0,0 +1,116 @@
+<?php
+/**
+ * ResponseHelper - Standardized JSON response formatting
+ *
+ * Provides consistent API response structure across all endpoints.
+ */
+class ResponseHelper {
+    /**
+     * Send a success response
+     *
+     * @param array $data Additional data to include
+     * @param string $message Success message
+     * @param int $code HTTP status code
+     */
+    public static function success($data = [], $message = 'Success', $code = 200) {
+        http_response_code($code);
+        header('Content-Type: application/json');
+        echo json_encode(array_merge([
+            'success' => true,
+            'message' => $message
+        ], $data));
+        exit;
+    }
+
+    /**
+     * Send an error response
+     *
+     * @param string $message Error message
+     * @param int $code HTTP status code
+     * @param array $data Additional data to include
+     */
+    public static function error($message, $code = 400, $data = []) {
+        http_response_code($code);
+        header('Content-Type: application/json');
+        echo json_encode(array_merge([
+            'success' => false,
+            'error' => $message
+        ], $data));
+        exit;
+    }
+
+    /**
+     * Send an unauthorized response (401)
+     *
+     * @param string $message Error message
+     */
+    public static function unauthorized($message = 'Authentication required') {
+        self::error($message, 401);
+    }
+
+    /**
+     * Send a forbidden response (403)
+     *
+     * @param string $message Error message
+     */
+    public static function forbidden($message = 'Access denied') {
+        self::error($message, 403);
+    }
+
+    /**
+     * Send a not found response (404)
+     *
+     * @param string $message Error message
+     */
+    public static function notFound($message = 'Resource not found') {
+        self::error($message, 404);
+    }
+
+    /**
+     * Send a validation error response (422)
+     *
+     * @param array $errors Validation errors
+     * @param string $message Error message
+     */
+    public static function validationError($errors, $message = 'Validation failed') {
+        self::error($message, 422, ['validation_errors' => $errors]);
+    }
+
+    /**
+     * Send a server error response (500)
+     *
+     * @param string $message Error message
+     */
+    public static function serverError($message = 'Internal server error') {
+        self::error($message, 500);
+    }
+
+    /**
+     * Send a rate limit exceeded response (429)
+     *
+     * @param int $retryAfter Seconds until retry is allowed
+     * @param string $message Error message
+     */
+    public static function rateLimitExceeded($retryAfter = 60, $message = 'Rate limit exceeded') {
+        header('Retry-After: ' . $retryAfter);
+        self::error($message, 429, ['retry_after' => $retryAfter]);
+    }
+
+    /**
+     * Send a created response (201)
+     *
+     * @param array $data Resource data
+     * @param string $message Success message
+     */
+    public static function created($data = [], $message = 'Resource created') {
+        self::success($data, $message, 201);
+    }
+
+    /**
+     * Send a no content response (204)
+     */
+    public static function noContent() {
+        http_response_code(204);
+        exit;
+    }
+}