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/schemas/image.py

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+import uuid
+from datetime import datetime
+
+from pydantic import BaseModel, ConfigDict, Field
+
+
+class ExamImageRead(BaseModel):
+    id: uuid.UUID
+    template_id: uuid.UUID
+    original_filename: str
+    stored_filename: str
+    mime_type: str
+    size_bytes: int
+    caption: str | None
+    content_url: str
+    created_at: datetime
+
+    model_config = ConfigDict(from_attributes=True)
+
+
+class ExamImageUploadResponse(BaseModel):
+    image: ExamImageRead
+    message: str = "Image uploaded successfully"
+
+
+class QuestionImageAttach(BaseModel):
+    image_id: uuid.UUID | None = Field(
+        default=None,
+        description="ID de imagen de la plantilla. null para desvincular.",
+    )