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/ticket_dependencies.php

@@ -64,6 +64,7 @@ if (session_status() === PHP_SESSION_NONE) {
 }
 
 require_once dirname(__DIR__) . '/config/config.php';
+require_once dirname(__DIR__) . '/helpers/Database.php';
 require_once dirname(__DIR__) . '/models/DependencyModel.php';
 require_once dirname(__DIR__) . '/models/AuditLogModel.php';
 require_once dirname(__DIR__) . '/helpers/ResponseHelper.php';
@@ -86,17 +87,8 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST' || $_SERVER['REQUEST_METHOD'] === 'DEL
     }
 }
 
-// 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();
 
 // Check if ticket_dependencies table exists
 $tableCheck = $conn->query("SHOW TABLES LIKE 'ticket_dependencies'");
@@ -211,6 +203,4 @@ switch ($method) {
     // Log detailed error server-side
     error_log('Ticket dependencies API error: ' . $e->getMessage() . ' in ' . $e->getFile() . ':' . $e->getLine());
     ResponseHelper::serverError('An error occurred while processing the dependency request');
-}
-
-$conn->close();
+};