safe move + sqlite WAL + log indexes + backup with logs

- fix: imap folder names with spaces (RFC3501 quoting in move/create/select)
- fix: move only deletes source after COPY + Message-ID verification in target
- fix: backup endpoint hung on sqlite write locks — enable WAL + busy_timeout
- perf: indexes on filter_logs(created_at, level, account_id+created_at) for
  fast log queries on millions of rows
- feat: optional "logs mit sichern" checkbox in backup export, restore on import
- UI: backup download uses fetch+blob with error display instead of location.href

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/services/scheduler.py

@@ -158,7 +158,7 @@ def _poll_account_sync(account_id: int) -> None:
                         try:
                             # Ordner muss ausgewählt sein
                             if not hasattr(client, '_current_folder') or client._current_folder != folder:
-                                client.conn.select(folder)
+                                client.select_folder(folder)
                                 client._current_folder = folder
                             mail = client.fetch_mail(uid)
                         except Exception as e:
@@ -195,7 +195,10 @@ def _poll_account_sync(account_id: int) -> None:
                         total_matched += 1
                         # Aktionen ausführen
                         for action in rule.actions:
-                            success = execute_action(client, mail, action, smtp_config)
+                            success = execute_action(
+                                client, mail, action, smtp_config,
+                                source_folder=folder,
+                            )
                             action_label = action.action_type.value
                             param = action.parameter or ""
                             if param: