Fix OpenAPI docs UI using hardcoded schema URL

The docs template always fetched from /schema.yml regardless of the
user's openapi_route setting. Now uses self.openapi_route so custom
schema paths (e.g. /openapi.json) work correctly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

responder/ext/openapi/__init__.py

@@ -216,7 +216,7 @@ class OpenAPISchema:
             f"{self.docs_theme}.html",
             title=self.title,
             version=self.version,
-            schema_url="/schema.yml",
+            schema_url=self.openapi_route,
         )
 
     def static_url(self, asset):