feat(zigbee): add MQTT TLS support with optional cert verify skip

- Schema: mqtt_tls + mqtt_tls_insecure flags on import + test-connection
  requests; model_validator enforces insecure requires tls
- Service: _build_tls_context() using ssl.create_default_context();
  logger.warning when verification disabled; tls_context plumbed into
  aiomqtt.Client for both fetch_networkmap and test_mqtt_connection
- Route: passes tls flags through to service
- Frontend: TLS checkbox auto-toggles port 1883<->8883 unless user
  edited; insecure checkbox disabled until TLS on, red-tinted; password
  field marked autocomplete=new-password
- Tests: 7 new backend tests (TLS context build, client kwargs assertion,
  router happy path, insecure-without-tls 422)

backend/app/api/routes/zigbee.py

@@ -38,6 +38,8 @@ async def import_zigbee_network(
             base_topic=payload.base_topic,
             username=payload.mqtt_username,
             password=payload.mqtt_password,
+            tls=payload.mqtt_tls,
+            tls_insecure=payload.mqtt_tls_insecure,
         )
     except ImportError as exc:
         raise HTTPException(status_code=500, detail=str(exc)) from exc
@@ -68,6 +70,8 @@ async def test_zigbee_connection(
             mqtt_port=payload.mqtt_port,
             username=payload.mqtt_username,
             password=payload.mqtt_password,
+            tls=payload.mqtt_tls,
+            tls_insecure=payload.mqtt_tls_insecure,
         )
         return ZigbeeTestConnectionResponse(connected=True, message="Connection successful")
     except ImportError as exc: