Add "truly done" ticket archiving with auto-unhide

- Let the user mark closed tickets as really done and hide them, so
  tickets closed prematurely by an agent stay visible until confirmed
- User-side "archived" flag stored in the DB (independent of Zammad),
  added via a schema migration on existing databases
- Hide archived tickets from the overview by default with an
  "erledigte einblenden" toggle; archive/unarchive from the ticket
  detail page and via a per-row quick action
- Poller automatically un-hides an archived ticket when new activity is
  detected, so nothing is missed

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/poller.py

@@ -196,8 +196,13 @@ def poll_once(client: ZammadClient) -> int:
                     except ZammadError:
                         pass
                 else:
-                    all_changes.extend(_detect_changes(conn, client, t, existing))
+                    ch = _detect_changes(conn, client, t, existing)
                     _upsert(conn, row, is_new=False)
+                    if ch:
+                        # Neue Aktivität -> ggf. abgelegtes Ticket wieder einblenden
+                        conn.execute(
+                            "UPDATE tickets SET archived=0 WHERE id=?", (t["id"],))
+                    all_changes.extend(ch)
         # Backfill NACH der Haupttransaktion (eigene, kurze Verbindungen)
         _backfill_articles(client)
     except ZammadError as e: