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/vin_decoder.py

@@ -0,0 +1,54 @@
+"""VIN Decoder using NHTSA vPIC API (free, no key needed)."""
+import re
+import requests
+
+
+def decode_vin(vin):
+    """Decode a VIN number using NHTSA API.
+    Returns: {make, model, year, engine, body_type, plant_country, fuel_type, error}
+    """
+    vin = (vin or "").strip().upper()
+
+    # Validate: 17 alphanumeric chars, no I/O/Q
+    if not re.match(r'^[A-HJ-NPR-Z0-9]{17}$', vin):
+        return {"error": "VIN debe tener exactamente 17 caracteres alfanumericos (sin I, O, Q)."}
+
+    url = f"https://vpic.nhtsa.dot.gov/api/vehicles/DecodeVinValues/{vin}?format=json"
+    resp = requests.get(url, timeout=10)
+    resp.raise_for_status()
+    data = resp.json()["Results"][0]
+
+    error_text = data.get("ErrorText", "") or ""
+    # NHTSA returns error codes like "0 - ..." for no error
+    has_real_error = error_text and not error_text.startswith("0")
+
+    displacement = data.get("DisplacementL", "") or ""
+    cylinders = data.get("EngineCylinders", "") or ""
+    engine_parts = []
+    if displacement:
+        engine_parts.append(f"{displacement}L")
+    if cylinders:
+        engine_parts.append(f"{cylinders}cyl")
+    engine_str = " ".join(engine_parts)
+
+    model_year = data.get("ModelYear", "")
+    year_int = None
+    if model_year:
+        try:
+            year_int = int(model_year)
+        except (ValueError, TypeError):
+            pass
+
+    return {
+        "vin": vin,
+        "make": data.get("Make", "") or "",
+        "model": data.get("Model", "") or "",
+        "year": year_int,
+        "engine": engine_str,
+        "body_type": data.get("BodyClass", "") or "",
+        "plant_country": data.get("PlantCountry", "") or "",
+        "fuel_type": data.get("FuelTypePrimary", "") or "",
+        "drive_type": data.get("DriveType", "") or "",
+        "trim": data.get("Trim", "") or "",
+        "error": error_text if has_real_error else "",
+    }