reminders: add recurring reminder support

Add cron expression support for recurring reminders using robfig/cron/v3.

Changes:
- Migration #2: ALTER TABLE reminders ADD COLUMN cron TEXT + next_fire_ts INTEGER
- Reminder struct: add Cron and NextFireTs fields
- scanReminder helper extracts full row including nullable cron
- CreateReminder: accept optional cron param, store next_fire_ts = fire_ts
- DueReminders: query on next_fire_ts instead of fire_ts
- RescheduleReminder: new method — parse cron, compute next fire, update
  next_fire_ts or mark fired if no more valid times
- Dispatcher: call RescheduleReminder for cron reminders, MarkReminder for
  one-shots (preserving existing behavior for ID=0 digest skip)
- ReminderCompleter interface: add RescheduleReminder method
- storeAPI adapter: forward RescheduleReminder
- All callers updated: CreateReminder signature includes cron param
- Tests: TestRecurringReminder (store), TestDispatchRecurringReminderReschedules
- Existing tests updated for new signature

internal/auth/auth_test.go

@@ -335,7 +335,7 @@ func (r *recordingAPI) Since(_ context.Context, _ string, _ time.Time) (time.Dur
 func (r *recordingAPI) Presence(_ context.Context) (ipc.Presence, error) {
 	return ipc.Presence{}, nil
 }
-func (r *recordingAPI) CreateReminder(_ context.Context, _ time.Time, _ string) (int64, error) {
+func (r *recordingAPI) CreateReminder(_ context.Context, _ time.Time, _, _ string) (int64, error) {
 	return 1, nil
 }
 func (r *recordingAPI) MarkReminder(_ context.Context, _ int64, _ string) error { return nil }