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/manage_workflows.php

@@ -8,6 +8,7 @@ ini_set('display_errors', 0);
 error_reporting(E_ALL);
 
 require_once dirname(__DIR__) . '/middleware/RateLimitMiddleware.php';
+require_once dirname(__DIR__) . '/models/WorkflowModel.php';
 RateLimitMiddleware::apply('api');
 
 try {
@@ -91,6 +92,7 @@ try {
             );
 
             if ($stmt->execute()) {
+                WorkflowModel::clearCache(); // Clear workflow cache
                 echo json_encode(['success' => true, 'transition_id' => $conn->insert_id]);
             } else {
                 echo json_encode(['success' => false, 'error' => $stmt->error]);
@@ -118,7 +120,11 @@ try {
                 $id
             );
 
-            echo json_encode(['success' => $stmt->execute()]);
+            $success = $stmt->execute();
+            if ($success) {
+                WorkflowModel::clearCache(); // Clear workflow cache
+            }
+            echo json_encode(['success' => $success]);
             $stmt->close();
             break;
 
@@ -130,7 +136,11 @@ try {
 
             $stmt = $conn->prepare("DELETE FROM status_transitions WHERE transition_id = ?");
             $stmt->bind_param('i', $id);
-            echo json_encode(['success' => $stmt->execute()]);
+            $success = $stmt->execute();
+            if ($success) {
+                WorkflowModel::clearCache(); // Clear workflow cache
+            }
+            echo json_encode(['success' => $success]);
             $stmt->close();
             break;