refactor: move all tests to backend/tests/ as black-box packages

All test files relocated from internal/X/ to tests/X/ and converted
to package X_test, using only the public API of each package.

- tests/auth/: jwt, service, handler integration tests
- tests/middleware/: auth, request_id, recovery tests
- tests/user/: calories, service, repository integration tests
- tests/locale/: locale tests (already package locale_test, just moved)
- tests/ingredient/: repository integration tests
- tests/recipe/: repository integration tests

mockUserRepo in tests/user/service_test.go redefined locally with
fully-qualified user.* types. Unexported auth.refreshRequest replaced
with a local testRefreshRequest struct in the integration test.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/tests/middleware/request_id_test.go

@@ -0,0 +1,61 @@
+package middleware_test
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/food-ai/backend/internal/middleware"
+)
+
+func TestRequestID_GeneratesNew(t *testing.T) {
+	handler := middleware.RequestID(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		id := middleware.RequestIDFromCtx(r.Context())
+		if id == "" {
+			t.Error("expected non-empty request ID in context")
+		}
+	}))
+
+	req := httptest.NewRequest("GET", "/", nil)
+	rr := httptest.NewRecorder()
+
+	handler.ServeHTTP(rr, req)
+	if rr.Header().Get("X-Request-ID") == "" {
+		t.Error("expected X-Request-ID in response header")
+	}
+}
+
+func TestRequestID_PreservesExisting(t *testing.T) {
+	handler := middleware.RequestID(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		id := middleware.RequestIDFromCtx(r.Context())
+		if id != "existing-id" {
+			t.Errorf("expected 'existing-id', got %q", id)
+		}
+	}))
+
+	req := httptest.NewRequest("GET", "/", nil)
+	req.Header.Set("X-Request-ID", "existing-id")
+	rr := httptest.NewRecorder()
+
+	handler.ServeHTTP(rr, req)
+	if rr.Header().Get("X-Request-ID") != "existing-id" {
+		t.Error("expected preserved X-Request-ID")
+	}
+}
+
+func TestRequestID_UniquePerRequest(t *testing.T) {
+	var ids []string
+	handler := middleware.RequestID(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		ids = append(ids, middleware.RequestIDFromCtx(r.Context()))
+	}))
+
+	for i := 0; i < 3; i++ {
+		req := httptest.NewRequest("GET", "/", nil)
+		rr := httptest.NewRecorder()
+		handler.ServeHTTP(rr, req)
+	}
+
+	if ids[0] == ids[1] || ids[1] == ids[2] {
+		t.Error("expected unique IDs for each request")
+	}
+}