Let PATCH household settings finish onboarding

PATCH /api/v1/settings/household accepts {"onboarding_completed": true},
which sets onboarding_completed_at to the update time (keeping the first
time once set). The timestamp stays read-only and false is refused, since
onboarding is never undone.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/server/settings.go

@@ -37,6 +37,9 @@ type settingsPatch struct {
 	SuggestionsTime  *string `json:"suggestions_time"`
 	ChatAutosendDow  *int    `json:"chat_autosend_dow"`
 	ChatAutosendTime *string `json:"chat_autosend_time"`
+	// OnboardingCompleted true sets onboarding_completed_at, which is
+	// itself read-only.
+	OnboardingCompleted *bool `json:"onboarding_completed"`
 }
 
 func getHouseholdSettings(logger *slog.Logger, svc *household.SettingsService) http.HandlerFunc {