small fix, don't worry about it

app/backend/internal/gateway/proxy.go

@@ -0,0 +1,85 @@
+package gateway
+
+import (
+	"bytes"
+	"io"
+	"net/http"
+	"net/url"
+	"strings"
+)
+
+func BuildTargetURL(upstreamURL, routePrefix, requestPath, rawQuery string) (string, error) {
+	base, err := url.Parse(upstreamURL)
+	if err != nil {
+		return "", err
+	}
+	trimmedPrefix := routePrefix
+	if trimmedPrefix == "/" {
+		trimmedPrefix = ""
+	}
+	withoutPrefix := requestPath
+	if trimmedPrefix != "" && strings.HasPrefix(requestPath, trimmedPrefix) {
+		withoutPrefix = strings.TrimPrefix(requestPath, trimmedPrefix)
+	}
+	if !strings.HasPrefix(withoutPrefix, "/") {
+		withoutPrefix = "/" + withoutPrefix
+	}
+	base.Path = strings.TrimRight(base.Path, "/") + withoutPrefix
+	base.RawQuery = rawQuery
+	return base.String(), nil
+}
+
+func ProxyRequest(client *http.Client, w http.ResponseWriter, r *http.Request, targetURL string, headers map[string]string) (int, []byte, error) {
+	var body []byte
+	if r.Body != nil {
+		defer r.Body.Close()
+		payload, err := io.ReadAll(r.Body)
+		if err != nil {
+			return 0, nil, err
+		}
+		body = payload
+	}
+
+	upstreamReq, err := http.NewRequestWithContext(r.Context(), r.Method, targetURL, bytes.NewReader(body))
+	if err != nil {
+		return 0, nil, err
+	}
+
+	for key, values := range r.Header {
+		lower := strings.ToLower(key)
+		if lower == "host" || lower == "content-length" || lower == "connection" {
+			continue
+		}
+		for _, value := range values {
+			upstreamReq.Header.Add(key, value)
+		}
+	}
+
+	for key, value := range headers {
+		upstreamReq.Header.Set(key, value)
+	}
+
+	res, err := client.Do(upstreamReq)
+	if err != nil {
+		return 0, nil, err
+	}
+	defer res.Body.Close()
+
+	for key, values := range res.Header {
+		if strings.EqualFold(key, "transfer-encoding") || strings.EqualFold(key, "connection") {
+			continue
+		}
+		for _, value := range values {
+			w.Header().Add(key, value)
+		}
+	}
+	w.WriteHeader(res.StatusCode)
+
+	respBody, err := io.ReadAll(res.Body)
+	if err != nil {
+		return res.StatusCode, nil, err
+	}
+	_, _ = w.Write(respBody)
+
+	return res.StatusCode, respBody, nil
+}