Change custom API path to /custom_api/ to avoid Gradio /api/ conflict

Gradio intercepts all /api/* routes when mounted at /. Use
/custom_api/digitize_base64 instead.

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

api_server.py

@@ -463,7 +463,7 @@ def create_fastapi_app():
         allow_headers=["*"],
     )
 
-    @app.post("/api/digitize_base64")
+    @app.post("/custom_api/digitize_base64")
     async def api_digitize_base64(request: Request):
         body = await request.json()
         data = body.get("data", [])