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/internal/recipe/repository_integration_test.go

@@ -1,35 +0,0 @@
-//go:build integration
-
-package recipe
-
-import (
-	"context"
-	"testing"
-
-	"github.com/food-ai/backend/internal/testutil"
-)
-
-func TestRecipeRepository_GetByID_NotFound(t *testing.T) {
-	pool := testutil.SetupTestDB(t)
-	repo := NewRepository(pool)
-	ctx := context.Background()
-
-	got, err := repo.GetByID(ctx, "00000000-0000-0000-0000-000000000000")
-	if err != nil {
-		t.Fatalf("unexpected error: %v", err)
-	}
-	if got != nil {
-		t.Error("expected nil for non-existent ID")
-	}
-}
-
-func TestRecipeRepository_Count(t *testing.T) {
-	pool := testutil.SetupTestDB(t)
-	repo := NewRepository(pool)
-	ctx := context.Background()
-
-	_, err := repo.Count(ctx)
-	if err != nil {
-		t.Fatalf("count: %v", err)
-	}
-}