fix(observers): separate packet metrics from presence updates

db/queries/observer_activity_summary.sql

@@ -0,0 +1,13 @@
+-- name: GetObserverActivityLiveSummary :one
+-- Two indexed ranges, bounded to one observer; no legacy presence counters.
+WITH latest AS (
+ SELECT heard_at FROM packet_observations
+ WHERE observer_id = @observer_id::uuid AND heard_at <= @generated_at::timestamptz
+ ORDER BY heard_at DESC LIMIT 1
+), hourly AS (
+ SELECT COUNT(*)::bigint AS n FROM packet_observations
+ WHERE observer_id = @observer_id::uuid
+ AND heard_at >= @hour_start::timestamptz AND heard_at < @hour_end::timestamptz
+)
+SELECT (SELECT heard_at FROM latest)::timestamptz AS latest_recorded_at,
+ hourly.n AS last_complete_hour FROM hourly;