feat: integrate AI grading functionality

- Added OpenAI package to backend dependencies for AI grading.
- Implemented new grading routes and services to handle AI grading requests.
- Updated admin settings to include AI provider configuration options.
- Enhanced frontend components to support AI grading, including UI for testing AI connection and displaying AI feedback.
- Updated results component to allow for AI-assisted grading and display of suggested grades.
- Added internationalization support for new AI grading features in both English and Dutch.

frontend/src/i18n/en.json

@@ -111,6 +111,15 @@
     "untimed": "Untimed"
   },
   "results": {
+    "ai": {
+      "button": "AI Review",
+      "suggestedGrade": "AI Suggested Grade",
+      "feedback": "AI Feedback",
+      "accept": "Accept",
+      "keepOriginal": "Keep original",
+      "rubricBreakdown": "Rubric breakdown",
+      "error": "AI grading failed — please grade manually"
+    },
     "reviewing": "Reviewing past attempt",
     "examComplete": "Exam complete",
     "scored": "You scored ",
@@ -205,6 +214,17 @@
     "storageWarningCritical": "You are using over 95% of your storage. Delete some items to free up space."
   },
   "admin": {
+    "aiGrading": {
+      "title": "AI Grading",
+      "provider": "Provider",
+      "model": "Model",
+      "apiKey": "API Key",
+      "baseUrl": "Base URL",
+      "baseUrlDesc": "Custom API base URL for OpenAI-compatible endpoints",
+      "testButton": "Test connection",
+      "testSuccess": "Connection successful",
+      "testFailed": "Connection failed"
+    },
     "administration": "Administration",
     "dashboard": "Dashboard",
     "backToSite": "← Back to site",