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/store/store_test.go

@@ -166,7 +166,7 @@ func TestRemindersRelativeResolvedAtCapture(t *testing.T) {
 	ctx := context.Background()
 	// capture path (router) converts "in 4h" → absolute. store just takes fire_ts.
 	fire := time.Now().UTC().Add(4 * time.Hour)
-	id, err := s.CreateReminder(ctx, fire, `{"text":"wake me"}`)
+	id, err := s.CreateReminder(ctx, fire, `{"text":"wake me"}`, "")
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -191,6 +191,53 @@ func TestRemindersRelativeResolvedAtCapture(t *testing.T) {
 	}
 }
 
+func TestRecurringReminder(t *testing.T) {
+	s := newTestStore(t)
+	ctx := context.Background()
+	now := time.Date(2026, 7, 1, 8, 0, 0, 0, time.UTC)
+
+	// Create a daily recurring reminder at 9:00
+	fire := time.Date(2026, 7, 1, 9, 0, 0, 0, time.UTC)
+	id, err := s.CreateReminder(ctx, fire, `{"text":"daily standup"}`, "0 9 * * *")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Not due yet (at 8:00, next_fire_ts = 9:00)
+	if due, err := s.DueReminders(ctx, now); err != nil || len(due) != 0 {
+		t.Fatalf("before fire: want 0 due, got %d (%v)", len(due), err)
+	}
+
+	// Due at 9:00
+	due, err := s.DueReminders(ctx, fire.Add(time.Second))
+	if err != nil || len(due) != 1 || due[0].ID != id {
+		t.Fatalf("after fire: want 1 due (%d), got %d (%v)", id, len(due), err)
+	}
+	if due[0].Cron != "0 9 * * *" {
+		t.Fatalf("cron: want %q, got %q", "0 9 * * *", due[0].Cron)
+	}
+
+	// Reschedule: next fire should be tomorrow 9:00
+	if err := s.RescheduleReminder(ctx, id, fire); err != nil {
+		t.Fatalf("RescheduleReminder: %v", err)
+	}
+	tomorrow := fire.Add(24 * time.Hour)
+	due, err = s.DueReminders(ctx, tomorrow.Add(time.Second))
+	if err != nil || len(due) != 1 || due[0].ID != id {
+		t.Fatalf("after reschedule: want 1 due (%d), got %d (%v)", id, len(due), err)
+	}
+
+	// Mark non-recurring reminder → ErrReminderState
+	_, err = s.CreateReminder(ctx, fire, `{"text":"one-shot"}`, "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	// The last id is id+1
+	if err := s.RescheduleReminder(ctx, id+1, fire); !errors.Is(err, ErrReminderState) {
+		t.Fatalf("reschedule one-shot: want ErrReminderState, got %v", err)
+	}
+}
+
 func TestNudgeOnceAndFeedbackOutcomes(t *testing.T) {
 	s := newTestStore(t)
 	ctx := context.Background()