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/core/config.py

@@ -22,6 +22,17 @@ class Settings(BaseSettings):
     jwt_algorithm: str = "HS256"
     jwt_expire_minutes: int = Field(default=60 * 24, ge=5)
     google_client_id: str | None = None
+    upload_dir: str = "/app/uploads"
+    max_upload_bytes: int = Field(default=20_971_520, ge=1_024)
+    max_materials_per_template: int = Field(default=10, ge=1, le=50)
+    max_reference_chars: int = Field(default=12_000, ge=1_000, le=100_000)
+    max_image_bytes: int = Field(default=5_242_880, ge=1_024)
+    max_images_per_template: int = Field(default=20, ge=1, le=100)
+    max_storage_bytes_per_template: int = Field(
+        default=52_428_800,
+        ge=1_024,
+        description="Cupo total por examen (materiales + imágenes). Por defecto 50 MB.",
+    )
 
     model_config = SettingsConfigDict(
         env_file=".env",