Tennessee WRAP backend pipeline update

What changed:
1. The backend now uses a general endpoint: POST /api/layer.
2. The request body includes { "layer": "topography", "feature": <GeoJSON Feature> }.
3. Topography processing now creates inspectable intermediate files.

Important generated files after clicking Topography:
backend/cache/<HUC>_<geometryHash>/
  request.json              records the request, HUC, bbox, export size, and artifact paths
  raw_dem.tif               real USGS 3DEP DEM for the watershed bounding box
  clipped_dem.tif           DEM clipped to the selected watershed polygon
  colored_elevation.png     rendered elevation color map used by Leaflet
  metadata.json             bounds, source, file paths, legend, raster shape
  statistics.json           min/mean/max/percentile elevation statistics

Run:
1. Double-click Start_WRAP.bat.
2. Open http://localhost:8000.
3. Select a watershed.
4. Click Topography.
5. Inspect backend/cache/... to verify the intermediate files.
