feat: vector flood-zone polygons replace the scanned-map overlay

Digitize the 7 Chiang Mai inundation zones from the municipal map into
geojson polygons: pixels classified by legend color, vectorized via
marching squares, and georeferenced by a 6-parameter affine fitted
least-squares to the OSM Ping centerline (109 m median residual after
outlier-filtered refits - the scanned image overlay could never scale
correctly and is removed).

The dashboard renders the zones as a Leaflet geoJSON layer styled live
from the forecast: fill opacity scales with each zone's 24 h exceedance
probability, zones whose trigger level the river has already reached get
a solid red border, and each polygon's popup shows its trigger level and
live probability. Zone styles refresh with every forecast load.

src/static/flood-zones-p1.geojson

@@ -0,0 +1 @@
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