Add React frontend and Sinbad2IA LLM integration.

Introduce a full Vite/React UI for exams, auth, materials, images, generation, and export.
Adapt backend for Sinbad2IA chat API, bcrypt passwords, CORS on port 5173, and schema migrations.

frontend/src/api/questions.js

@@ -0,0 +1,8 @@
+import { api } from "./client";
+
+export async function attachImageToQuestion(questionId, imageId) {
+  const { data } = await api.patch(`/exam/questions/${questionId}/image`, {
+    image_id: imageId,
+  });
+  return data; // QuestionRead
+}