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/migrations.go

@@ -34,6 +34,7 @@ var migrations = []migration{
 	{11, "E-Mail-Bestätigung", (*Store).migrateEmailVerification},
 	{12, "Markierung für Einträge von KI-Agenten", (*Store).migrateAgentMark},
 	{13, "Herkunft von Einträgen (API-Schlüssel)", (*Store).migrateOrigin},
+	{14, "Erledigte Aufgaben aufräumen", (*Store).migrateDoneCleanup},
 }
 
 func (s *Store) migrate() error {