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/material.py

@@ -0,0 +1,32 @@
+import uuid
+from datetime import datetime
+
+from pydantic import BaseModel, ConfigDict, Field
+
+from app.models.exam import MaterialStatus
+
+
+class ExamMaterialRead(BaseModel):
+    id: uuid.UUID
+    template_id: uuid.UUID
+    original_filename: str
+    mime_type: str
+    size_bytes: int
+    status: MaterialStatus
+    error_message: str | None
+    text_preview: str | None = None
+    created_at: datetime
+
+    model_config = ConfigDict(from_attributes=True)
+
+
+class ExamMaterialUploadResponse(BaseModel):
+    material: ExamMaterialRead
+    message: str = "File uploaded and processed successfully"
+
+
+class MaterialIdsFilter(BaseModel):
+    material_ids: list[uuid.UUID] | None = Field(
+        default=None,
+        description="Si se indica, solo se usan estos materiales como contexto.",
+    )