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/models/property.py

@@ -51,6 +51,8 @@ class Property(db.Model):
         index=True,
     )
     created_at = db.Column(db.DateTime, nullable=False, server_default=db.func.now())
+    video_url = db.Column(db.String(512), nullable=True)
+    video_position = db.Column(db.String(20), nullable=False, server_default='section')
 
     photos = db.relationship(
         "PropertyPhoto",