feat(admin): add 7 new admin plugins
- unpinall: unpin all messages via api.unpin_all_chat_messages
- chatinfo: /settitle and /setdescription for chat metadata
- customtitle: set admin custom titles (16 char limit)
- slowmode: control slow mode delay (off/10s/30s/1m/5m/15m/1h)
- topic: forum topic management (create/close/reopen/delete/rename)
- joinrequest: configurable join request policy (auto-approve, auto-reject, manual with approve/reject buttons); hooks into on_chat_join_request router event
- chatpermissions: inline keyboard UI to toggle default chat permissions with live updates via set_chat_permissions