Add materials, exam images, storage quota, and API guide

Upload documents for AI context, exam images for Moodle questions, per-template storage limits, embedded images in XML export, and GUIA_API_Y_FLUJO.md with full endpoint documentation.

backend/app/api/routes/questions.py

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+import uuid
+from typing import Annotated
+
+from fastapi import APIRouter, Depends
+
+from app.api.dependencies import get_exam_service, get_image_service
+from app.core.auth import get_current_user
+from app.models.user import User
+from app.schemas.exam import QuestionRead
+from app.schemas.image import QuestionImageAttach
+from app.services.exam_service import ExamService
+from app.services.image_service import ImageService
+
+router = APIRouter(prefix="/questions", tags=["questions"])
+
+
+@router.patch("/{question_id}/image", response_model=QuestionRead)
+def attach_image_to_question(
+    question_id: uuid.UUID,
+    payload: QuestionImageAttach,
+    current_user: Annotated[User, Depends(get_current_user)],
+    exam_service: Annotated[ExamService, Depends(get_exam_service)],
+    image_service: Annotated[ImageService, Depends(get_image_service)],
+) -> QuestionRead:
+    question, template = exam_service.get_owned_question(current_user.id, question_id)
+    updated = image_service.attach_image_to_question(template, question, payload.image_id)
+    return exam_service.to_question_read(updated)