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/check_duplicates.php

@@ -11,6 +11,7 @@ RateLimitMiddleware::apply('api');
 
 session_start();
 require_once dirname(__DIR__) . '/config/config.php';
+require_once dirname(__DIR__) . '/helpers/Database.php';
 require_once dirname(__DIR__) . '/helpers/ResponseHelper.php';
 
 header('Content-Type: application/json');
@@ -32,17 +33,8 @@ 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');
-}
+// Use centralized database connection
+$conn = Database::getConnection();
 
 // Search for similar titles
 // Use both LIKE for substring matching and SOUNDEX for phonetic matching
@@ -112,6 +104,4 @@ usort($duplicates, function($a, $b) {
 // Limit to top 5
 $duplicates = array_slice($duplicates, 0, 5);
 
-$conn->close();
-
 ResponseHelper::success(['duplicates' => $duplicates]);