feat(backend): add video_url and video_position to properties

- Model: video_url VARCHAR(512) nullable, video_position VARCHAR(20) default 'section'
- Migration: k3l4m5n6o7p8_add_video_to_properties
- Admin route: expose/accept video_url (sanitize empty->NULL) and video_position
- Public schema: PropertyDetailOut exposes both fields

backend/app/schemas/property.py

@@ -61,3 +61,5 @@ class PropertyDetailOut(PropertyOut):
     address: str | None = None
     code: str | None = None
     description: str | None = None
+    video_url: str | None = None
+    video_position: Literal['carousel', 'section'] = 'section'