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/api/gradeApi.ts

@@ -0,0 +1,21 @@
+import { apiFetch } from '../utils/api.js'
+
+export interface GradingResult {
+  suggestedGrade: number
+  feedback: string
+  detail?: unknown
+}
+
+export interface GradeRequest {
+  type: 'essay' | 'short-answer' | 'translation' | 'math' | 'ordering' | 'match'
+  question: Record<string, unknown>
+  answer: unknown
+  language: string
+}
+
+export function gradeQuestion(req: GradeRequest): Promise<GradingResult> {
+  return apiFetch('/api/grade', {
+    method: 'POST',
+    body: JSON.stringify(req),
+  })
+}