feat(web-svc-translate): answer the canonical root with the documentation

The gateway defined no route for /, so FastAPI answered its own canonical
domain with a JSON 404. Its only browser surface is /docs, which the proxy
already injects logout.js into, so the universal-logout spec read an empty
surface and failed on a role whose logout wiring was in fact correct.

web-opt-rdr-domains opts out of the tor contract as a surfaceless role
(c4e64656be). That shape does not fit here: this gateway has a surface, it was
just not where its canonical URL pointed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

roles/web-svc-translate/files/python/translate_gateway/app.py

@@ -5,7 +5,7 @@
 from __future__ import annotations
 
 from fastapi import FastAPI, Request
-from fastapi.responses import JSONResponse
+from fastapi.responses import JSONResponse, RedirectResponse
 
 from translate_gateway.errors import GatewayError, NoBackendError
 
@@ -32,6 +32,10 @@ def create_app(gateway, engines, *, detector=None):
             return detector
         return next((e for e in engines.values() if hasattr(e, "detect")), None)
 
+    @app.get("/", include_in_schema=False)
+    async def _root() -> RedirectResponse:
+        return RedirectResponse(url="/docs")
+
     @app.exception_handler(GatewayError)
     async def _refuse(_request: Request, exc: GatewayError) -> JSONResponse:
         """Answer an error rather than the untranslated source text.