Stamp a funnel column (shown/opened/skipped/dismissed) — first-write-wins. skipped also increments skip_count.

gate_declined is a DIAGNOSTIC event (not a funnel column): the client fires it once per run when a survey is targeted + cached but the gate still won't show it (reason = already_shown / skipped_this_session). Log-only so we can see, fleet-wide, WHY a targeted survey stays hidden — the delivery-debug signal added with the cache-warmup fix. sessions_lt_min:x/y was a third reason until #2758 removed the threshold; clients that predate it can still send it, so the handler keeps accepting any reason string rather than validating against a list.