Chat autosend, week rollover and ai_runs purge schedules (Feature 20)

shopping_chat_autosend fires at chat_autosend_dow/time for the coming
week and posts its Supply run through SendToChat when a webhook is
stored, list_ready's Chat channel is on and the list exists unsent;
otherwise it logs why (key list_ready:chat:{weekStart}). week_rollover
freezes the previous week at Monday 00:05 through Freeze, judging the
week ended by the fire time. ai_runs_purge calls harness.Purge daily at
03:00 with the fire time less RunRetention. Fake-clock tests cover each.

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

internal/notify/notify.go

@@ -27,6 +27,7 @@ import (
 	"fmt"
 	"log/slog"
 	"net/url"
+	"slices"
 	"strings"
 	"sync"
 	"time"
@@ -238,6 +239,19 @@ func (s *Service) emitTx(ctx context.Context, tx *sql.Tx, ev Event, channels fun
 	return nil
 }
 
+// ChannelOn reports whether the preference of event type ev switches
+// channel ch on. A type with no preference row is an error.
+func (s *Service) ChannelOn(ctx context.Context, ev EventType, ch Channel) (bool, error) {
+	pref, err := db.New(s.st.Reader).GetNotificationPref(ctx, string(ev))
+	if errors.Is(err, sql.ErrNoRows) {
+		return false, fmt.Errorf("notify: no notification preference for %s", ev)
+	}
+	if err != nil {
+		return false, fmt.Errorf("notify: read notification preference for %s: %w", ev, err)
+	}
+	return slices.Contains(enabledChannels(pref), ch), nil
+}
+
 // enabledChannels returns the channels pref switches on.
 func enabledChannels(pref db.NotificationPref) []Channel {
 	var chs []Channel