fix: resolve CI failures — gofmt, unused functions, missing test helpers

- Run gofmt on user schema, config test, group handler
- Remove unused mergeGroupIDs function
- Restore shared test helpers (newJSONResponse, queuedHTTPUpstream)
  that were in deleted Sora test file

backend/internal/service/account_test_helpers_test.go

@@ -0,0 +1,48 @@
+package service
+
+import (
+	"fmt"
+	"io"
+	"net/http"
+	"strings"
+
+	"github.com/Wei-Shaw/sub2api/internal/pkg/tlsfingerprint"
+)
+
+// queuedHTTPUpstream is a test helper that serves pre-loaded responses in order.
+type queuedHTTPUpstream struct {
+	responses []*http.Response
+	requests  []*http.Request
+	tlsFlags  []bool
+}
+
+func (u *queuedHTTPUpstream) Do(_ *http.Request, _ string, _ int64, _ int) (*http.Response, error) {
+	return nil, fmt.Errorf("unexpected Do call")
+}
+
+func (u *queuedHTTPUpstream) DoWithTLS(req *http.Request, _ string, _ int64, _ int, profile *tlsfingerprint.Profile) (*http.Response, error) {
+	u.requests = append(u.requests, req)
+	u.tlsFlags = append(u.tlsFlags, profile != nil)
+	if len(u.responses) == 0 {
+		return nil, fmt.Errorf("no mocked response")
+	}
+	resp := u.responses[0]
+	u.responses = u.responses[1:]
+	return resp, nil
+}
+
+// newJSONResponse creates a simple HTTP response for testing.
+func newJSONResponse(status int, body string) *http.Response {
+	return &http.Response{
+		StatusCode: status,
+		Header:     make(http.Header),
+		Body:       io.NopCloser(strings.NewReader(body)),
+	}
+}
+
+// newJSONResponseWithHeader creates a JSON response with a custom header.
+func newJSONResponseWithHeader(status int, body, key, value string) *http.Response {
+	resp := newJSONResponse(status, body)
+	resp.Header.Set(key, value)
+	return resp
+}