feat(2-E): GDACS disaster adapter

Adds the GDACS (Global Disaster Alert and Coordination System) adapter
against the self-describing framework. Polls https://www.gdacs.org/xml/rss.xml
every 600s, parses the RSS items, and publishes to a new CENTRAL_DISASTER
JetStream stream on central.disaster.<eventtype_lower>.<country_lower>.

Locked decisions:
- Keep: WF, DR, FL, VO, TC. Drop: EQ (USGS canonical on central.quake.>),
  plus any future-unknown eventtype.
- Filter via settings_schema event_types: list[str] so operators can
  re-allow without a code change.
- Dedup by RSS guid (format <eventtype><eventid>, stable across reissue).
- Severity from gdacs:alertlevel (Green=1, Orange=2, Red=3, default 0).
- Fall-off uses GDACS gdacs:iscurrent=false as explicit tombstone signal,
  with a fallback for items that vanish entirely from the feed. Tombstones
  publish on disaster.removed.<eventtype>.<country>.
- Geo: centroid from geo:Point, bbox from gdacs:bbox (reordered to Geo
  (minLon, minLat, maxLon, maxLat)), primary_region from gdacs:iso3.

CENTRAL_DISASTER stream: 7d retention, 1 GiB max_bytes, mirroring
CENTRAL_FIRE / CENTRAL_QUAKE / CENTRAL_SPACE. Migrations 020 (adapter row,
enabled=false, default event_types in settings) and 021 (stream seed).
STREAM_SUBJECTS, archive STREAMS, GUI DASHBOARD_STREAMS each pick up
the new stream.

Tests: 14 new in tests/test_gdacs.py using frozen RSS fixtures with WF/DR/EQ/XX
items (covering normalization, EQ drop, unknown drop, settings override,
guid dedup, iscurrent=false tombstone, missing-from-feed tombstone,
helper boundaries). Stream-count assertions bumped 4->5 and 5->6 for
the new stream (anti-pattern noted; queued as a follow-up PR E.5).
+1 membership test test_streams_contains_central_disaster.
Full suite: 426 passed.

End-to-end on CT104: 48 events published on first poll (44 disaster.wf +
4 disaster.fl), zero EQ events, all subjects under central.disaster.>
with lowercase-hyphenated country suffixes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_dashboard.py

@@ -205,7 +205,7 @@ class TestDashboardStreamsIsolation:
         call_args = mock_templates.TemplateResponse.call_args
         context = call_args.kwargs.get("context", call_args[1].get("context"))
         streams = context["streams"]
-        assert len(streams) == 5
+        assert len(streams) == 6
         fire_stream = next(s for s in streams if s["name"] == "CENTRAL_FIRE")
         assert fire_stream.get("error") == "unavailable"
         wx_stream = next(s for s in streams if s["name"] == "CENTRAL_WX")