_win_printf skipped buffer/pad append for WIN_CHAT while the user was
scrolled, relying on chatwin_db_history() to replay missed messages on
scroll-down. That replay is unavailable when PREF_DBLOG is "off" (no
write) or "redact" ([REDACTED] body), or when the backend failed to
initialize — in those cases the message was lost.
Introduce log_database_can_recover_messages() and gate the WIN_CHAT
early return on it: when recovery is impossible, fall through and
append to the buffer just like non-chat windows do. Also treat
DB_RESPONSE_ERROR in win_page_down the same as DB_RESPONSE_EMPTY so
the [SCROLLED] indicator doesn't hang when the backend is dead.