Enhance API routing and testing for user endpoints

- Updated the reverse proxy to redirect `GET` requests for `/api/{alias}/users` to `/v1/stats/users`, ensuring compatibility with Telemt builds that handle these requests differently.
- Added tests to verify that `GET` and `HEAD` requests to `/api/{alias}/users` are correctly rewritten, while `POST` requests and user-specific retrievals remain unchanged.
- Improved documentation in GATEWAY_RUN.md to clarify the behavior of the API routing and the importance of using the correct base URL.

internal/proxy/reverse.go

@@ -9,7 +9,11 @@ import (
 
 // NewReverseProxy builds a reverse proxy to target base URL with path rewriting:
 // stripPrefix (/api/{alias}) + pathPrefix (/v1) + remainder, joined onto target via url.JoinPath
-// (e.g. https://host/api/ + v1 + users → https://host/api/v1/users).
+// (e.g. https://host/api/ + v1 + health → https://host/v1/health).
+//
+// Compatibility: GET/HEAD .../api/{alias}/users (list only, no extra path segment) is sent upstream as
+// /v1/stats/users. Some Telemt builds treat GET /v1/users incorrectly (e.g. 400) while /v1/stats/users works.
+// POST .../users (create) and GET .../users/{username} are unchanged.
 func NewReverseProxy(target *url.URL, stripPrefix, pathPrefix string, setAuth string) *httputil.ReverseProxy {
 	proxy := httputil.NewSingleHostReverseProxy(target)
 	proxy.Transport = DirectTransport()
@@ -30,6 +34,9 @@ func NewReverseProxy(target *url.URL, stripPrefix, pathPrefix string, setAuth st
 		}
 		rest := strings.TrimPrefix(p, stripPrefix)
 		rest = strings.TrimPrefix(rest, "/")
+		if (req.Method == http.MethodGet || req.Method == http.MethodHead) && rest == "users" {
+			rest = "stats/users"
+		}
 		joined := buildUpstreamURL(target, pathPrefix, rest)
 		req.URL.Scheme = joined.Scheme
 		req.URL.Host = joined.Host