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/migrations/versions/k3l4m5n6o7p8_add_video_to_properties.py

@@ -0,0 +1,26 @@
+"""add video to properties
+
+Revision ID: k3l4m5n6o7p8
+Revises: j2k3l4m5n6o7
+Create Date: 2026-04-22
+"""
+import sqlalchemy as sa
+from alembic import op
+
+revision = 'k3l4m5n6o7p8'
+down_revision = 'j2k3l4m5n6o7'
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column('properties',
+        sa.Column('video_url', sa.String(512), nullable=True))
+    op.add_column('properties',
+        sa.Column('video_position', sa.String(20),
+                  nullable=False, server_default='section'))
+
+
+def downgrade() -> None:
+    op.drop_column('properties', 'video_position')
+    op.drop_column('properties', 'video_url')