feat: cascade-delete hosts when removing a network

When a VLAN/LAN is deleted, all non-gateway, non-livebox devices
with an interface in that network are deleted automatically.
Gateway and livebox devices are preserved; their interface is
unlinked (vlan_id set to NULL).

The confirmation dialog now shows the exact count of devices
that will be deleted (all three locales: fr/en/es).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/routers/vlans.py

@@ -97,6 +97,21 @@ def delete_vlan(vlan_pk: int, db: Session = Depends(get_db)):
     db_vlan = db.query(models.Vlan).filter(models.Vlan.id == vlan_pk).first()
     if not db_vlan:
         raise HTTPException(status_code=404, detail="VLAN introuvable")
+
+    # Collect devices with an interface in this VLAN
+    ifaces = (
+        db.query(models.DeviceInterface)
+        .filter(models.DeviceInterface.vlan_id == vlan_pk)
+        .all()
+    )
+    device_ids = {i.device_id for i in ifaces}
+
+    for device_id in device_ids:
+        device = db.query(models.Device).filter(models.Device.id == device_id).first()
+        if device and not device.is_gateway and not device.is_livebox:
+            db.delete(device)  # cascade deletes all its interfaces
+
+    # Gateway/livebox interfaces in this VLAN will be SET NULL by SQLAlchemy
     db.delete(db_vlan)
     db.commit()
     return {"ok": True}