perf(booth): optimize assets and result flow

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

lib/watermark.py

@@ -17,7 +17,7 @@ def watermark_portrait(image_path_or_buffer):
     bw, bh = base.size
 
     # 1. School Logo (Top-Left)
-    logo_path = os.path.join(process_cwd(), "public", "assets", "brand", "shanghai_karate_logo.png")
+    logo_path = os.path.join(process_cwd(), "public", "assets", "brand", "shanghai_karate_logo.webp")
     if os.path.exists(logo_path):
         logo = Image.open(logo_path).convert("RGBA")
         target_logo_w = int(bw * 0.28)