fix(pos): retry con backoff para rate limit 429 de OpenRouter

3 reintentos con espera 5s/10s/15s. Si agota reintentos,
muestra mensaje amigable en vez de error.

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

pos/services/ai_chat.py

@@ -60,6 +60,10 @@ def chat(user_message, conversation_history=None):
         messages.extend(conversation_history)
     messages.append({"role": "user", "content": user_message})
 
+    import time
+    max_retries = 3
+
+    for attempt in range(max_retries):
         try:
             resp = requests.post(
                 OPENROUTER_URL,
@@ -73,19 +77,24 @@ def chat(user_message, conversation_history=None):
                     "max_tokens": 500,
                     "temperature": 0.3,
                 },
-            timeout=15,
+                timeout=20,
             )
+            if resp.status_code == 429:
+                # Rate limited — wait and retry
+                wait = (attempt + 1) * 5  # 5s, 10s, 15s
+                if attempt < max_retries - 1:
+                    time.sleep(wait)
+                    continue
+                return {"message": "El asistente está ocupado. Intenta de nuevo en unos segundos.", "search_query": None, "vehicle": None}
             resp.raise_for_status()
             data = resp.json()
             content = data["choices"][0]["message"]["content"]
 
             # Try to parse JSON response
             try:
-            # Handle markdown-wrapped JSON (```json ... ```)
                 stripped = content.strip()
                 if stripped.startswith("```"):
                     lines = stripped.split("\n")
-                # Remove first and last lines (``` markers)
                     json_str = "\n".join(lines[1:-1])
                     parsed = json.loads(json_str)
                 else:
@@ -94,6 +103,8 @@ def chat(user_message, conversation_history=None):
             except (json.JSONDecodeError, IndexError):
                 return {"message": content, "search_query": None, "vehicle": None}
         except Exception as e:
+            if attempt < max_retries - 1:
+                continue
             return {
                 "message": f"Error de conexion: {str(e)}",
                 "search_query": None,