fix(pos): corregir model ID — qwen/qwen3.6-plus-preview:free

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pos/services/ai_chat.py

@@ -10,7 +10,7 @@ OPENROUTER_URL = "https://openrouter.ai/api/v1/chat/completions"
 # ⚠️ SOLO MODELOS GRATUITOS — No cambiar a modelos de pago.
 # El modelo DEBE terminar en ":free" para garantizar costo $0.
 # Alternativas gratuitas: "meta-llama/llama-4-scout:free", "google/gemma-3-27b-it:free"
-MODEL = "qwen/qwen3-6b-preview:free"
+MODEL = "qwen/qwen3.6-plus-preview:free"
 
 def _validate_model(model_id):
     """Ensure only free models are used. Raises if model is not free."""