fix: MiMo ASR 认证头改为 Authorization: Bearer

MiMo API 使用标准 Bearer Token 认证，而非自定义 api-key 头。

backend/internal/ai/stt/mimo_test.go

@@ -21,8 +21,8 @@ func newTestMiMoService(handler http.HandlerFunc) (*MiMoService, *httptest.Serve
 func TestMiMoService_Recognize_Success(t *testing.T) {
 	s, srv := newTestMiMoService(func(w http.ResponseWriter, r *http.Request) {
 		// 验证请求
-		if r.Header.Get("api-key") != "test-key" {
-			t.Errorf("expected api-key test-key, got %s", r.Header.Get("api-key"))
+		if r.Header.Get("Authorization") != "Bearer test-key" {
+			t.Errorf("expected Authorization Bearer test-key, got %s", r.Header.Get("Authorization"))
 		}
 		if r.URL.Path != "/chat/completions" {
 			t.Errorf("expected path /chat/completions, got %s", r.URL.Path)