Fix snapshot NameError and crop zone bbox for PlateRecognizer

Add missing subprocess import in main.py (snapshot was crashing).
When a zone is defined, crop each frame to the zone's bounding box
before sending to PlateRecognizer — smaller payload, faster upload,
recognition focused on the relevant area only.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/main.py

@@ -5,6 +5,7 @@ import logging
 import threading
 import tempfile
 import asyncio
+import subprocess
 from contextlib import asynccontextmanager
 from fastapi import FastAPI, Request, Query, HTTPException, UploadFile, File, Form
 from fastapi.responses import HTMLResponse, RedirectResponse