Enforce calibration-first workflow: block detection and VAR until calibrated

- Remove fallback corner estimation (_estimate_corners_from_net)
- Gate YOLO detection behind calibration check in detection_loop
- Add calibration overlay UI with prominent Calibrate button
- Disable Court/Trajectory tabs until system_ready
- Skip trajectory/VAR/event polling when uncalibrated
- Add system_ready flag to calibration status API

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

src/web/app.py

@@ -100,7 +100,9 @@ def api_calibration_status():
     cals = {}
     for sid, cal in state.get('calibrators', {}).items():
         cals[str(sid)] = cal.calibrated
-    return jsonify(cals)
+    # System is ready when at least one camera is calibrated
+    any_calibrated = any(cals.values()) if cals else False
+    return jsonify({**cals, 'system_ready': any_calibrated})
 
 
 @app.route('/api/calibration/trigger', methods=['POST'])