feat(tasks): completed tasks are deleted after a delay (default 30 days), "Erledigte leeren"

thoughts.completed_at is set by triggers on every way a task is completed (migration 14; tasks
completed earlier count from the update). Users choose the delay (7/30/90/365 days or never,
users.done_retention_days); for tasks in a list the owner's delay counts. An hourly job deletes
expired completed tasks (not in the demo) and tells their viewers. POST /tasks/clear-done clears
completed tasks the user may edit. The done tab shows the setting and the button. v0.35.0.

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

internal/storage/done_cleanup.go

@@ -0,0 +1,148 @@
+package storage
+
+import (
+	"errors"
+	"strings"
+	"time"
+)
+
+// Completed tasks are deleted a while after they were ticked off, so the "Erledigt" tab does not fill
+// up for good. Every user chooses the delay (done_retention_days, 0 = never); for tasks in a task list
+// the owner's choice counts, for personal tasks the user's own. thoughts.completed_at is kept by two
+// triggers, so every way of completing a task (app, Kanban, note checkbox, API) sets it.
+
+// DoneRetentionChoices are the delays offered in the app, in days; 0 keeps completed tasks.
+var DoneRetentionChoices = []int{0, 7, 30, 90, 365}
+
+const DefaultDoneRetentionDays = 30
+
+var ErrInvalidRetention = errors.New("Ungültige Frist für erledigte Aufgaben (erlaubt: 0, 7, 30, 90, 365 Tage)")
+
+func (s *Store) migrateDoneCleanup() error {
+	if err := s.addColumnIfMissing("users", "done_retention_days", "INTEGER NOT NULL DEFAULT 30"); err != nil {
+		return err
+	}
+	if err := s.addColumnIfMissing("thoughts", "completed_at", "TEXT NOT NULL DEFAULT ''"); err != nil {
+		return err
+	}
+	return s.execInTx(
+		// Tasks completed before this update count from now on, so nothing disappears at once
+		stmt(`UPDATE thoughts SET completed_at = strftime('%Y-%m-%dT%H:%M:%SZ', 'now') WHERE is_completed = 1 AND completed_at = ''`),
+		stmt(`CREATE TRIGGER IF NOT EXISTS trg_thoughts_completed AFTER UPDATE OF is_completed ON thoughts
+			WHEN NEW.is_completed != OLD.is_completed BEGIN
+				UPDATE thoughts SET completed_at = CASE WHEN NEW.is_completed = 1 THEN strftime('%Y-%m-%dT%H:%M:%SZ', 'now') ELSE '' END
+				WHERE id = NEW.id;
+			END`),
+		stmt(`CREATE TRIGGER IF NOT EXISTS trg_thoughts_completed_insert AFTER INSERT ON thoughts
+			WHEN NEW.is_completed = 1 AND NEW.completed_at = '' BEGIN
+				UPDATE thoughts SET completed_at = strftime('%Y-%m-%dT%H:%M:%SZ', 'now') WHERE id = NEW.id;
+			END`),
+	)
+}
+
+// DoneRetentionDays: after how many days the user's completed tasks are deleted (0 = never).
+func (s *Store) DoneRetentionDays(userID string) int {
+	days := DefaultDoneRetentionDays
+	_ = s.db.QueryRow(`SELECT done_retention_days FROM users WHERE id = ?`, userID).Scan(&days)
+	return days
+}
+
+// SetDoneRetentionDays stores one of DoneRetentionChoices; other values are refused.
+func (s *Store) SetDoneRetentionDays(userID string, days int) error {
+	ok := false
+	for _, d := range DoneRetentionChoices {
+		ok = ok || d == days
+	}
+	if !ok {
+		return ErrInvalidRetention
+	}
+	_, err := s.db.Exec(`UPDATE users SET done_retention_days = ? WHERE id = ?`, days, userID)
+	return err
+}
+
+// DeletedTask: what callers need to tell everyone who saw a deleted task.
+type DeletedTask struct {
+	ID, UserID, TaskListID string
+}
+
+// Completed tasks: a kept task, a converted task or a task linked to a note checkbox
+const completedTaskSQL = `t.is_completed = 1 AND (t.status = 'kept' OR (t.status = 'converted' AND t.converted_type = 'task'))`
+
+// PurgeExpiredDoneTasks deletes completed tasks whose delay has run out at now: the task list owner's
+// delay, or the user's own for personal tasks. A note checkbox linked to such a task stays as a plain
+// checkbox (the link lives on the task).
+func (s *Store) PurgeExpiredDoneTasks(now time.Time) ([]DeletedTask, error) {
+	rows, err := s.db.Query(`
+		SELECT t.id, t.user_id, t.task_list_id
+		FROM thoughts t
+		LEFT JOIN task_lists tl ON tl.id = t.task_list_id AND t.task_list_id != ''
+		JOIN users u ON u.id = COALESCE(tl.user_id, t.user_id)
+		WHERE `+completedTaskSQL+`
+		  AND t.completed_at != '' AND u.done_retention_days > 0
+		  AND t.completed_at < strftime('%Y-%m-%dT%H:%M:%SZ', ?, '-' || u.done_retention_days || ' days')`,
+		now.UTC().Format("2006-01-02 15:04:05"))
+	if err != nil {
+		return nil, err
+	}
+	return s.deleteTasks(rows)
+}
+
+// ClearDoneTasks deletes the completed tasks of one task list ("" = all lists the user can see) that
+// the user may delete: own personal tasks and tasks in lists they own or edit.
+func (s *Store) ClearDoneTasks(userID, taskListID string) ([]DeletedTask, error) {
+	if taskListID != "" {
+		if err := s.ensureTaskListWritable(userID, taskListID); err != nil {
+			return nil, err
+		}
+		rows, err := s.db.Query(`SELECT t.id, t.user_id, t.task_list_id FROM thoughts t
+			WHERE `+completedTaskSQL+` AND t.task_list_id = ?`, taskListID)
+		if err != nil {
+			return nil, err
+		}
+		return s.deleteTasks(rows)
+	}
+	lists, err := s.GetTaskLists(userID)
+	if err != nil {
+		return nil, err
+	}
+	writable := []any{userID}
+	marks := []string{}
+	for _, l := range lists {
+		if l.Permission != "viewer" {
+			writable = append(writable, l.ID)
+			marks = append(marks, "?")
+		}
+	}
+	inLists := ""
+	if len(marks) > 0 {
+		inLists = ` OR t.task_list_id IN (` + strings.Join(marks, ",") + `)`
+	}
+	rows, err := s.db.Query(`SELECT t.id, t.user_id, t.task_list_id FROM thoughts t
+		WHERE `+completedTaskSQL+` AND ((t.user_id = ? AND t.task_list_id = '')`+inLists+`)`, writable...)
+	if err != nil {
+		return nil, err
+	}
+	return s.deleteTasks(rows)
+}
+
+// deleteTasks deletes the tasks the query rows name and returns them.
+func (s *Store) deleteTasks(rows interface {
+	Next() bool
+	Scan(dest ...any) error
+	Close() error
+}) ([]DeletedTask, error) {
+	var out []DeletedTask
+	for rows.Next() {
+		var d DeletedTask
+		if rows.Scan(&d.ID, &d.UserID, &d.TaskListID) == nil {
+			out = append(out, d)
+		}
+	}
+	_ = rows.Close()
+	for _, d := range out {
+		if _, err := s.db.Exec(`DELETE FROM thoughts WHERE id = ?`, d.ID); err != nil {
+			return nil, err
+		}
+	}
+	return out, nil
+}