feat(coverage): nationwide USGS NED fallback fixes non-Texas sites

The 3DEP+TNRIS catalog was Texas-only; coverage compute crashed for
any site outside the catalog with 'No LIDAR terrain data available
for this site'. Production hit this for at least one user today.

This commit lays in the rest of the cnHeat-style backend foundation:

- Towerops.Lidar.Sources.UsgsNed: synthesizes Tile structs for the
  USGS National Elevation Dataset (1/3 arc-second / ~10 m), keyed
  off the predictable n{lat:02d}w{lon:03d} 1° tile naming on the
  prd-tnm S3 bucket. Covers all of CONUS, AK, HI, and PR/USVI.
- Towerops.Lidar: when the curated catalog has no tile, falls back
  to the synthetic NED tiles via the same Reader / GDAL /vsicurl/
  pipeline. New :lidar_ned_fallback config flag (default on);
  test_helper.exs disables it so tests can still assert :no_tile
  without spinning up GDAL stubs.
- coverage_worker error message reflects the new geographic scope.
- coverage_buildings table + Towerops.Coverages.Building schema:
  scaffolding for the next sprint where Microsoft Global ML
  Building Footprints get imported and woven into the path
  profile as DSM clutter.
- Tests: usgs_ned_test for tile_for_point/2 (CONUS, HI, AK, PR,
  out-of-extent); grid_test fallback path verifying the synthetic
  URL is hit and an AAIGrid response composes correctly.

test/towerops/workers/coverage_worker_test.exs

@@ -90,7 +90,7 @@ defmodule Towerops.Workers.CoverageWorkerTest do
 
       reloaded = Coverages.get_coverage!(org.id, coverage.id)
       assert reloaded.status == "failed"
-      assert reloaded.error_message =~ "No LIDAR terrain data"
+      assert reloaded.error_message =~ "No terrain data"
     end
 
     test "refuses to run when org_id in args mismatches the coverage's org",