feat: allow interfaces to have no zone (shorewall '-' zone)

backend/alembic/versions/0005_interface_zone_nullable.py

@@ -0,0 +1,21 @@
+"""make interface zone_id nullable
+
+Revision ID: 0005
+Revises: 0004
+Create Date: 2026-03-01
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = "0005"
+down_revision = "0004"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.alter_column("interfaces", "zone_id", existing_type=sa.Integer(), nullable=True)
+
+
+def downgrade() -> None:
+    op.alter_column("interfaces", "zone_id", existing_type=sa.Integer(), nullable=False)