feat(pos): add inventory-aware chat context (#31) and VIN decoder (#17)

Chat now fetches tenant inventory summary (brands, counts, low-stock)
and injects it into the AI system prompt so responses prioritize local
stock. VIN decoder uses free NHTSA vPIC API to decode 17-char VINs and
auto-fills the vehicle selector dropdowns when a catalog match is found.

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

pos/services/ai_chat.py

@@ -89,10 +89,80 @@ El search_query SIEMPRE debe ser en ingles (el catalogo TecDoc esta en ingles).
 """
 
 
-def chat(user_message, conversation_history=None):
-    """Send a message to the AI and get a response with search suggestions."""
+def get_inventory_context(tenant_conn, branch_id=None):
+    """Build a summary string of the tenant's inventory for AI context.
+
+    Returns a string like:
+        Este negocio tiene 1234 productos en inventario.
+        Categorias: BOSCH (45), MONROE (32), ACDelco (28), ...
+        Productos con stock bajo (<=3): 15
+    """
+    cur = tenant_conn.cursor()
+    try:
+        # Total items
+        where = "i.is_active = true"
+        params = []
+        if branch_id:
+            where += " AND i.branch_id = %s"
+            params.append(branch_id)
+
+        cur.execute(f"SELECT COUNT(*) FROM inventory i WHERE {where}", params)
+        total = cur.fetchone()[0] or 0
+
+        if total == 0:
+            return "CONTEXTO DEL INVENTARIO:\nEste negocio aun no tiene productos en inventario."
+
+        # Top brands with counts
+        cur.execute(f"""
+            SELECT i.brand, COUNT(*) as cnt
+            FROM inventory i
+            WHERE {where} AND i.brand IS NOT NULL AND i.brand != ''
+            GROUP BY i.brand
+            ORDER BY cnt DESC
+            LIMIT 15
+        """, params)
+        brands = cur.fetchall()
+        brand_list = ", ".join(f"{row[0]} ({row[1]})" for row in brands if row[0])
+
+        # Products with low stock (<=3)
+        cur.execute(f"""
+            SELECT COUNT(*) FROM inventory i
+            WHERE {where}
+              AND COALESCE((SELECT SUM(quantity) FROM inventory_operations WHERE inventory_id = i.id), 0) <= 3
+        """, params)
+        low_stock = cur.fetchone()[0] or 0
+
+        lines = [
+            "CONTEXTO DEL INVENTARIO:",
+            f"Este negocio tiene {total} productos en inventario.",
+        ]
+        if brand_list:
+            lines.append(f"Marcas disponibles: {brand_list}")
+        lines.append(f"Productos con stock bajo (<=3 unidades): {low_stock}")
+        lines.append("IMPORTANTE: Cuando busques partes, SIEMPRE prioriza lo que el negocio tiene en inventario local.")
+
+        return "\n".join(lines)
+    except Exception:
+        return ""
+    finally:
+        cur.close()
+
+
+def chat(user_message, conversation_history=None, inventory_context=None):
+    """Send a message to the AI and get a response with search suggestions.
+
+    Args:
+        user_message: The user's chat message.
+        conversation_history: Previous messages in the conversation.
+        inventory_context: Optional inventory summary string to inject into the system prompt.
+    """
     _validate_model(MODEL)  # Block paid models
-    messages = [{"role": "system", "content": SYSTEM_PROMPT}]
+
+    system_content = SYSTEM_PROMPT
+    if inventory_context:
+        system_content = SYSTEM_PROMPT + "\n\n" + inventory_context
+
+    messages = [{"role": "system", "content": system_content}]
     if conversation_history:
         messages.extend(conversation_history)
     messages.append({"role": "user", "content": user_message})