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/api/done_cleanup.go

@@ -0,0 +1,59 @@
+package api
+
+import (
+	"encoding/json"
+	"log/slog"
+	"net/http"
+	"time"
+
+	"github.com/sandrodach/mindsift/internal/auth"
+	"github.com/sandrodach/mindsift/internal/storage"
+)
+
+// startDoneCleanupTicker deletes completed tasks whose delay ran out (storage/done_cleanup.go), once
+// shortly after the start and then every hour.
+func (s *Server) startDoneCleanupTicker() {
+	time.Sleep(2 * time.Minute)
+	s.runDoneCleanup(time.Now())
+	ticker := time.NewTicker(time.Hour)
+	defer ticker.Stop()
+	for now := range ticker.C {
+		s.runDoneCleanup(now)
+	}
+}
+
+func (s *Server) runDoneCleanup(now time.Time) {
+	deleted, err := s.store.PurgeExpiredDoneTasks(now)
+	if err != nil {
+		slog.Error("Aufräumen erledigter Aufgaben fehlgeschlagen", "error", err)
+		return
+	}
+	if len(deleted) > 0 {
+		slog.Info("Erledigte Aufgaben aufgeräumt", "count", len(deleted))
+	}
+	s.broadcastDeletedTasks(deleted)
+}
+
+// broadcastDeletedTasks tells everyone who saw the tasks; there are no watcher notifications for this.
+func (s *Server) broadcastDeletedTasks(deleted []storage.DeletedTask) {
+	for _, d := range deleted {
+		audience := append(s.store.GetTaskListUserIDs(d.TaskListID), d.UserID)
+		s.wsHub.BroadcastToUsers(audience, "TASK_DELETED", map[string]string{"id": d.ID})
+	}
+}
+
+// handleClearDoneTasks: "Erledigte leeren" for one task list (taskListId) or all the user can edit.
+func (s *Server) handleClearDoneTasks(w http.ResponseWriter, r *http.Request) {
+	userID := auth.GetUserID(r.Context())
+	var req struct {
+		TaskListID string `json:"taskListId"`
+	}
+	_ = json.NewDecoder(r.Body).Decode(&req)
+	deleted, err := s.store.ClearDoneTasks(userID, req.TaskListID)
+	if err != nil {
+		writeStoreError(w, http.StatusBadRequest, err)
+		return
+	}
+	s.broadcastDeletedTasks(deleted)
+	writeJSON(w, http.StatusOK, map[string]int{"deleted": len(deleted)})
+}