feat(tasks): repeating tasks (daily, weekly, monthly, yearly)

- recurrence on tasks, set in the detail dialog; 🔁 on rows and Kanban cards
- Completing a repeating task (detail, list, Kanban drag to done, note
  checkbox) creates the next one: title, tags, priority, notes, list,
  assignees and rule copied, subtasks reset, first open column, due date
  one interval later (month end clamped); pushed live as TASK_CREATED
- Each task spawns at most once (re-open + complete makes no duplicate)
- Export/import keep the rule (Obsidian Tasks "🔁 every week", JSON field)

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

internal/storage/recurrence.go

@@ -0,0 +1,129 @@
+package storage
+
+import (
+	"errors"
+	"time"
+
+	"github.com/sandrodach/mindsift/internal/domain"
+)
+
+// ErrInvalidRecurrence is returned for unknown repeat rules.
+var ErrInvalidRecurrence = errors.New("Unbekannte Wiederholung (erlaubt: täglich, wöchentlich, monatlich, jährlich)")
+
+func validRecurrence(r string) bool {
+	switch r {
+	case "", "daily", "weekly", "monthly", "yearly":
+		return true
+	}
+	return false
+}
+
+// NextDueDate moves a due date (YYYY-MM-DD, or today when empty) one interval forward. Months and years
+// keep the day where possible and fall back to the month's last day (31 Jan -> 28/29 Feb).
+func NextDueDate(due, rule string, today time.Time) string {
+	base, err := time.Parse("2006-01-02", due)
+	if err != nil {
+		base = time.Date(today.Year(), today.Month(), today.Day(), 0, 0, 0, 0, time.UTC)
+	}
+	switch rule {
+	case "daily":
+		base = base.AddDate(0, 0, 1)
+	case "weekly":
+		base = base.AddDate(0, 0, 7)
+	case "monthly":
+		base = addMonthsClamped(base, 1)
+	case "yearly":
+		base = addMonthsClamped(base, 12)
+	}
+	return base.Format("2006-01-02")
+}
+
+func addMonthsClamped(t time.Time, months int) time.Time {
+	first := time.Date(t.Year(), t.Month(), 1, 0, 0, 0, 0, time.UTC).AddDate(0, months, 0)
+	last := first.AddDate(0, 1, -1).Day()
+	day := t.Day()
+	if day > last {
+		day = last
+	}
+	return time.Date(first.Year(), first.Month(), day, 0, 0, 0, 0, time.UTC)
+}
+
+// SetTaskRecurrence changes the repeat rule of a task (editors of its list or its owner).
+func (s *Store) SetTaskRecurrence(userID, taskID, rule string) error {
+	if !validRecurrence(rule) {
+		return ErrInvalidRecurrence
+	}
+	t, err := s.GetThoughtByID(userID, taskID)
+	if err != nil {
+		return err
+	}
+	if err := s.ensureTaskListWritable(userID, t.TaskListID); err != nil {
+		return err
+	}
+	_, err = s.db.Exec(`UPDATE thoughts SET recurrence = ? WHERE id = ?`, rule, taskID)
+	return err
+}
+
+func (s *Store) getThoughtRaw(id string) (*domain.Thought, error) {
+	t, err := scanThought(s.db.QueryRow(
+		`SELECT id, user_id, content, tags, priority, source, status, is_completed, converted_type, converted_ref_id, due_date, task_list_id, kanban_status, notes, alarm_sent, created_at, triaged_at, subtasks, sort_order, recurrence
+		 FROM thoughts WHERE id = ?`, id))
+	if err != nil {
+		return nil, err
+	}
+	t.Assignees = s.getTaskAssignees(t.ID)
+	return t, nil
+}
+
+// spawnNextOccurrence creates the follow-up of a repeating task that was just completed: same title,
+// tags, priority, notes, list, assignees and rule, subtasks unchecked, due date one interval later,
+// in the list's first open column. Each task spawns at most once, so re-opening and completing it
+// again creates no duplicate. The new task goes to OnTaskCreated (live update for all clients).
+func (s *Store) spawnNextOccurrence(taskID string) {
+	t, err := s.getThoughtRaw(taskID)
+	if err != nil || t.Recurrence == "" || !t.IsCompleted {
+		return
+	}
+	res, err := s.db.Exec(`UPDATE thoughts SET recurrence_spawned = 1 WHERE id = ? AND recurrence_spawned = 0`, taskID)
+	if err != nil {
+		return
+	}
+	if n, _ := res.RowsAffected(); n == 0 {
+		return // already spawned its successor
+	}
+
+	column := "todo"
+	if t.TaskListID != "" {
+		if tl, err := s.GetTaskListByID(t.UserID, t.TaskListID); err == nil {
+			for _, c := range tl.Columns {
+				if !c.IsDone {
+					column = c.ID
+					break
+				}
+			}
+		}
+	}
+	due := NextDueDate(t.DueDate, t.Recurrence, time.Now())
+	next, err := s.CreateThoughtWithDetails(t.UserID, t.Content, t.Priority, t.Source, due, t.TaskListID, column, t.Notes)
+	if err != nil {
+		return
+	}
+	if _, err := s.UpdateThoughtTriage(t.UserID, next.ID, "keep", "task", "", due, t.TaskListID); err != nil {
+		return
+	}
+	subs := make([]domain.SubTask, len(t.Subtasks))
+	for i, st := range t.Subtasks {
+		subs[i] = domain.SubTask{ID: domain.NewID("st"), Title: st.Title, CreatedAt: domain.NowISO()}
+	}
+	tags := t.Tags
+	_, _ = s.UpdateThoughtDetails(t.UserID, next.ID, nil, nil, nil, nil, nil, &column, nil, &subs, &tags)
+	_, _ = s.db.Exec(`UPDATE thoughts SET recurrence = ?, sort_order = ? WHERE id = ?`, t.Recurrence, t.SortOrder, next.ID)
+	for _, uid := range t.Assignees {
+		_, _ = s.db.Exec(`INSERT OR IGNORE INTO task_assignees (task_id, user_id, created_at) VALUES (?, ?, ?)`, next.ID, uid, domain.NowISO())
+	}
+	s.PruneTaskAssignees()
+
+	if created, err := s.getThoughtRaw(next.ID); err == nil && s.OnTaskCreated != nil {
+		s.OnTaskCreated(created)
+	}
+}