fix(nws): replace centroid filter with polygon intersection

- Add shapely dependency for geometry intersection
- Replace _point_in_region with _geometry_intersects_region
- Uses Shapely shape() and box() for proper GeoJSON handling
- Avoids false negatives on large alert polygons

Also adds antimeridian-crossing bbox rejection to RegionConfig validator.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/central/config_models.py

@@ -22,6 +22,8 @@ class RegionConfig(BaseModel):
             )
         if self.east == self.west:
             raise ValueError("east and west cannot be equal (zero-width bbox)")
+        if self.east < self.west:
+            raise ValueError("antimeridian-crossing bboxes not supported")
         return self