fix(usgs_quake): stop dropping first-sighted quakes at the freshness gate

The USGS feed is a rolling PAST-DAY feed (2.5_day.geojson), so a
genuinely first-seen quake's age-at-ingest routinely exceeds the
generic per-toggle freshness_seconds (600s) window, silently dropping
it downstream of decide()/DB-insert (last_broadcast_at stayed NULL on
every sampled first-sighting row).

- dispatcher.py: earthquake_event gets its own adapter_config-backed
  freshness override (adapter_config.usgs_quake.freshness_seconds,
  default 3600s), scoped to the CATEGORY (not the "seismic" toggle,
  which also carries hydro stream_flood_warning/stream_high_water and
  must keep the generic per-toggle freshness).
- usgs_quake.py: to_event() no longer passes the adapter's fixed
  freshness in, deferring to the dispatcher override.
- adapter_config/defaults.py: adds the freshness_seconds field/default
  for usgs_quake.

work/tests/test_adapter_config_api.py

@@ -86,6 +86,7 @@ def test_per_adapter_list(client):
         "regional_centroid", "regional_radius_mi",
         "broadcast_pager_alerts", "global_mag_floor",
         "regional_mag_floor", "escalate_mag_floor",
+        "freshness_seconds",
     }