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/domain/models.go

@@ -141,6 +141,8 @@ type User struct {
 	EmailUnverified bool `json:"emailUnverified,omitempty"`
 	// Only filled for the user's own profile (GET /auth/me, PATCH /auth/settings)
 	NotificationSettings *NotificationSettings `json:"notificationSettings,omitempty"`
+	// DoneRetentionDays: completed tasks are deleted this many days after ticking them off (0 = never)
+	DoneRetentionDays int `json:"doneRetentionDays"`
 }
 
 // NotificationSettings: which events reach a user over which channel, the daily summary and a pause.