fn_notify_applog_err 404 signature collapse — fleet-wide alert-dedup bug

fn_notify_applog_err signature SQL: left(COALESCE(NULLIF(errorMessage,''), detail->>'digest', action), 500) collapses every page_not_found/404 into one signature bucket (errorMessage NULL + no detail.digest on nav rows -> falls through to literal action string). Any dedup/alert layer grouping by signature misreports a brand-new 404 on an unrelated route as a recurrence of an old one. Fix: key page_not_found/nav signatures off route/path instead of action, or carry URL in the notify payload. Fleet-wide bug (pm-pluto-cc discovery via nw-venus-cc), confirmed byte-identical in mars migration 012-...-applog-notify-add-useragent.sql. Class-A (trigger function DDL) — pre-implementation audit design review required.