Add performance, security, and reliability improvements

- Consolidate all 20 API files to use centralized Database helper
- Add optimistic locking to ticket updates to prevent concurrent conflicts
- Add caching to StatsModel (60s TTL) for dashboard performance
- Add health check endpoint (api/health.php) for monitoring
- Improve rate limit cleanup with cron script and efficient DirectoryIterator
- Enable rate limit response headers (X-RateLimit-*)
- Add audit logging for workflow transitions
- Log Discord webhook failures instead of silencing
- Fix visibility check on export_tickets.php
- Add database migration system with performance indexes
- Fix cron recurring tickets to use assignTicket method

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

api/upload_attachment.php

@@ -19,6 +19,7 @@ if (session_status() === PHP_SESSION_NONE) {
 }
 
 require_once dirname(__DIR__) . '/config/config.php';
+require_once dirname(__DIR__) . '/helpers/Database.php';
 require_once dirname(__DIR__) . '/helpers/ResponseHelper.php';
 require_once dirname(__DIR__) . '/models/AttachmentModel.php';
 require_once dirname(__DIR__) . '/models/AuditLogModel.php';
@@ -171,28 +172,20 @@ try {
     }
 
     // Log the upload
-    $conn = new mysqli(
-        $GLOBALS['config']['DB_HOST'],
-        $GLOBALS['config']['DB_USER'],
-        $GLOBALS['config']['DB_PASS'],
-        $GLOBALS['config']['DB_NAME']
+    $conn = Database::getConnection();
+    $auditLog = new AuditLogModel($conn);
+    $auditLog->log(
+        $_SESSION['user']['user_id'],
+        'attachment_upload',
+        'ticket_attachments',
+        (string)$attachmentId,
+        [
+            'ticket_id' => $ticketId,
+            'filename' => $originalFilename,
+            'size' => $file['size'],
+            'mime_type' => $mimeType
+        ]
     );
-    if (!$conn->connect_error) {
-        $auditLog = new AuditLogModel($conn);
-        $auditLog->log(
-            $_SESSION['user']['user_id'],
-            'attachment_upload',
-            'ticket_attachments',
-            (string)$attachmentId,
-            [
-                'ticket_id' => $ticketId,
-                'filename' => $originalFilename,
-                'size' => $file['size'],
-                'mime_type' => $mimeType
-            ]
-        );
-        $conn->close();
-    }
 
     ResponseHelper::created([
         'attachment_id' => $attachmentId,