feat(mapset): HotSpot legend + Risico mapset attachment (#919)

sql/migrate/add_hotspot_layer.sql

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+-- HotSpot map layer for the Risico mapset (issue Laixer/FunderMaps#919).
+--
+-- A HotSpot is a building whose HIGHEST risk class across the two
+-- groundwater-driven models is D or E:
+--
+--     MAX(drystand_risk, dewatering_depth_risk) >= 'd'
+--
+-- The classes are ordered a < b < c < d < e, so "the maximum" is simply the
+-- worse of the two; A/B/C are excluded entirely. The layer is a fast visual
+-- answer to "where should we look first", not a new model.
+--
+-- No data work is needed: maplayer.building_tiles already carries both
+-- drystand_risk and dewatering_depth_risk, and maplayer.buildings(z,x,y)
+-- emits both at every zoom (z12-13 included), so the whole layer is a
+-- MapLibre filter + paint expression over the existing 'buildings' tile
+-- source. Styling lives in FunderMapsWebFront src/config/layers/hotspot.json
+-- and must stay in sync with the legend below.
+--
+-- Scope on 2026-07-26: 165,286 of 6,449,590 buildings (2.6%) qualify -
+-- 8,817 with a worst class of E, 156,469 with D.
+
+--------------------------------------------------------------------------------
+-- 1. Legend. Two entries, reusing the exact D and E colors of the existing
+--    drystand-risk / dewatering-depth-risk legends so the HotSpot layer reads
+--    as a summary of those two rather than a separate scale.
+--    "order" = 0 puts HotSpot at the top of the Risico legend, above the
+--    individual model layers it summarizes.
+--------------------------------------------------------------------------------
+
+INSERT INTO application.mapset_layer (id, name, fields, "order") VALUES (
+    'hotspot',
+    'HotSpot',
+    '[
+        {"name": "Verhoogd risico (D)", "color": "FFAC33"},
+        {"name": "Hoog risico (E)",     "color": "FF5533"}
+    ]'::jsonb,
+    0
+)
+ON CONFLICT (id) DO UPDATE SET name = EXCLUDED.name, fields = EXCLUDED.fields;
+
+--------------------------------------------------------------------------------
+-- 2. Attach to the Risico mapset.
+--------------------------------------------------------------------------------
+
+UPDATE application.mapset
+SET layers = array_append(layers, 'hotspot')
+WHERE id = 'clcqkaaea000714o1v1jmdqwf'
+  AND NOT ('hotspot' = ANY (layers));