fix cache

CACHE_SETUP.md

@@ -76,3 +76,17 @@ Run local code checks:
 python3 -m unittest discover -s tests -p 'test_*.py' -v
 python3 tests/smoke_cache_routes.py
 ```
+
+## Duplicate Date and RustProxy 0.29.3
+
+`/convert` deliberately does not set its own `Date` header after
+`Response.make_conditional()`. Werkzeug can create a `Date` header there;
+Uvicorn also supplies one, and RustProxy 0.29.3 rejects multiple `Date`
+headers with `response_ttl_size_or_metadata`.
+
+Verify that **exactly one** `Date` header is returned directly by the
+ASGI backend (no reverse proxy in the path). All success responses should
+also include a valid `Content-Length`, `ETag` and public `Cache-Control`.
+For a full seven days, set RustProxy global `max_ttl_seconds` to `604800`
+AND configure the app's `CACHE_S_MAXAGE=604800` in `.env`.
+The `unsafe_force` cache switch is not required for `/convert`.