Add caching layer and database helper

- Add CacheHelper for file-based caching with TTL support
- Add Database helper for centralized connection management
- Update WorkflowModel to cache status transitions (10 min TTL)
- Update UserPreferencesModel to cache user prefs (5 min TTL)
- Update manage_workflows.php to clear cache on changes
- Update get_users.php to use Database helper (example migration)

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

api/get_users.php

@@ -12,6 +12,7 @@ RateLimitMiddleware::apply('api');
 
 try {
     require_once dirname(__DIR__) . '/config/config.php';
+    require_once dirname(__DIR__) . '/helpers/Database.php';
 
     // Check authentication (session already started by RateLimitMiddleware)
     if (!isset($_SESSION['user']) || !isset($_SESSION['user']['user_id'])) {
@@ -20,22 +21,10 @@ try {
         exit;
     }
 
-    $conn = new mysqli(
-        $GLOBALS['config']['DB_HOST'],
-        $GLOBALS['config']['DB_USER'],
-        $GLOBALS['config']['DB_PASS'],
-        $GLOBALS['config']['DB_NAME']
-    );
-
-    if ($conn->connect_error) {
-        throw new Exception("Database connection failed");
-    }
-
     header('Content-Type: application/json');
 
     // Get all users for mentions/assignment
-    $sql = "SELECT user_id, username, display_name FROM users ORDER BY display_name, username";
-    $result = $conn->query($sql);
+    $result = Database::query("SELECT user_id, username, display_name FROM users ORDER BY display_name, username");
 
     if (!$result) {
         throw new Exception("Failed to query users");
@@ -52,8 +41,6 @@ try {
 
     echo json_encode(['success' => true, 'users' => $users]);
 
-    $conn->close();
-
 } catch (Exception $e) {
     http_response_code(500);
     echo json_encode(['success' => false, 'error' => $e->getMessage()]);