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

backend/app/schemas.py

@@ -64,7 +64,7 @@ class ZoneOut(BaseModel):
 # --- Interface ---
 class InterfaceCreate(BaseModel):
     name: str
-    zone_id: int
+    zone_id: Optional[int] = None
     options: str = ""
 
 
@@ -78,7 +78,7 @@ class InterfaceOut(BaseModel):
     id: int
     config_id: int
     name: str
-    zone_id: int
+    zone_id: Optional[int]
     options: str
 
     model_config = {"from_attributes": True}