Upstream endpoint should always end with a /

[mostlygeek]

When requesting /upstream/model the upstream UI should redirect to /upstream/model/. llama-swap's UI already creates upstream links with a trailing /. However, when manually requesting /upstream/:model it should respond with a 301 (Moved Permanently) to /upstream/:model/.

The current behaviour causes problems with llama-server's legacy UI as well as the new UI with hash based routing (ggml-org/llama.cpp#16157).

[isaac-mcfadyen]

Oh thank you for pointing this out! I noticed during the switch to hash-based routing but thought it was something to do with the new UI and not just an edge case that could be fixed here.