perf(booth): optimize assets and result flow

WebP reduces UI payload; JPEG keeps portrait saves compatible.\nFinish ends the visit without prompting another portrait.

watermark_processor.py

@@ -8,7 +8,7 @@ def apply_watermark(input_img_path, output_img_path):
     bw, bh = base.size
 
     base_dir = os.path.dirname(os.path.abspath(__file__))
-    logo_path = os.path.join(base_dir, "public", "assets", "brand", "shanghai_karate_logo.png")
+    logo_path = os.path.join(base_dir, "public", "assets", "brand", "shanghai_karate_logo.webp")
     qr_path = os.path.join(base_dir, "public", "assets", "brand", "dojo_qr.png")
 
     margin = int(bw * 0.035)