Add detection zone configuration with polygon editor

New /config/zone page lets users draw a polygon over a camera frame
(including live RTSP snapshot) to define the area where vehicle detection
applies. Zone is stored in /data/zone.json and applied in two ways:
- Motion scoring: inter-frame diff is masked outside the zone so garden
  movement doesn't inflate frame scores
- YOLO plate detection: detections whose center falls outside the zone
  are filtered out

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

app/templates/index.html

@@ -35,6 +35,7 @@
       <span class="text-slate-400 text-sm">{{ total }} passage{{ 's' if total != 1 else '' }}</span>
       <a href="/stats" class="btn-upload" style="text-decoration:none;">📊 Stats</a>
       <a href="/whitelist" class="btn-upload" style="text-decoration:none;">🛡 Whitelist</a>
+      <a href="/config/zone" class="btn-upload" style="text-decoration:none;">⬡ Zone</a>
       <button class="btn-upload" onclick="document.getElementById('upload-panel').classList.toggle('open')">⬆ Tester un clip</button>
     </div>
   </header>