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.

backend/app/db/init_db.py

@@ -1,7 +1,9 @@
 from app.db.base import Base
+from app.db.migrations import run_migrations
 from app.db.session import engine
 from app.models import exam, user  # noqa: F401
 
 
 def init_db() -> None:
     Base.metadata.create_all(bind=engine)
+    run_migrations(engine)