Same tick previously fired every due watch back-to-back (burst of N requests,
worst right after a restart when all watches are due at once). Now each watch
URL fetch is followed by a pause of FETCH_GAP_SECONDS (float, default 1, env
tunable; 0 disables) on both success and failure paths.
Also thread kw_fk_col into load_ignored_keywords() inside
send_pending_notifications() so the physical-FK lookup (445040a) actually takes
effect instead of silently falling back to the watch Link column.
hoelee