Fix all static analysis errors (golangci-lint)

- Fix errcheck: handle all error return values in production code
- Fix errcheck: handle all error return values in test files
- Fix staticcheck: replace deprecated WithCredentialsFile with WithAuthCredentialsFile
- Remove unused code: authHeaders, planToEatPlannerItem, planToEatResponse
- Use defer func() { _ = x.Close() }() pattern for ignored close errors

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/internal/api/google_calendar.go b/internal/api/google_calendar.go
index dc61f3d..e4f9c2f 100644
--- a/internal/api/google_calendar.go
+++ b/internal/api/google_calendar.go
@@ -55,7 +55,8 @@ func deduplicateEvents(events []models.CalendarEvent) []models.CalendarEvent {
 // NewGoogleCalendarClient creates a client that fetches from multiple calendars.
 // calendarIDs can be comma-separated (e.g., "cal1@group.calendar.google.com,cal2@group.calendar.google.com")
 func NewGoogleCalendarClient(ctx context.Context, credentialsFile, calendarIDs string) (*GoogleCalendarClient, error) {
-	srv, err := calendar.NewService(ctx, option.WithCredentialsFile(credentialsFile))
+	// Use type-safe credential loading (replaces deprecated WithCredentialsFile)
+	srv, err := calendar.NewService(ctx, option.WithAuthCredentialsFile(option.ServiceAccount, credentialsFile))
 	if err != nil {
 		return nil, fmt.Errorf("unable to retrieve Calendar client: %v", err)
 	}