fix(coverage): use gdalwarp for elevation grid + sanitise UI errors

gdal_translate doesn't accept -t_srs (only -projwin_srs for input),
so reprojection silently fell over against any non-WGS84 source COG
(3DEP Albers tiles). Switch to gdalwarp which actually reprojects.

Also wrap raw GDAL stderr behind a generic user-facing message and
log the technical detail server-side — operators shouldn't see the
gdal_translate usage banner pasted into a coverage failure card.

lib/towerops/lidar/reader.ex

@@ -58,26 +58,30 @@ defmodule Towerops.Lidar.Reader do
   @spec elevation_grid(Tile.t(), {number(), number(), number(), number()}, number()) ::
           {:ok, map()} | {:error, term()}
   def elevation_grid(%Tile{url: url}, {west, south, east, north}, cell_size_deg) do
+    # gdalwarp reprojects to EPSG:4326 on the fly. gdal_translate cannot
+    # reproject — it only crops in the source CRS — so 3DEP Albers tiles
+    # would otherwise come back in projected metres.
     args = [
       "-of",
       "AAIGrid",
-      "-projwin_srs",
+      "-t_srs",
       "EPSG:4326",
-      "-projwin",
+      "-te",
       to_string(west),
-      to_string(north),
-      to_string(east),
       to_string(south),
+      to_string(east),
+      to_string(north),
       "-tr",
       to_string(cell_size_deg),
       to_string(cell_size_deg),
-      "-t_srs",
-      "EPSG:4326",
+      "-r",
+      "bilinear",
+      "-overwrite",
       vsicurl(url),
       "/vsistdout/"
     ]
 
-    case run("gdal_translate", args) do
+    case run("gdalwarp", args) do
       {output, 0} -> parse_aaigrid(output)
       {output, code} -> {:error, {:gdal, code, output}}
     end

lib/towerops/workers/coverage_worker.ex

@@ -431,7 +431,15 @@ defmodule Towerops.Workers.CoverageWorker do
 
   defp describe_error({:terrain_unavailable, :nodata}), do: "LIDAR tiles for this area returned no data."
 
-  defp describe_error({:terrain_unavailable, reason}), do: "Terrain fetch failed: #{inspect(reason)}"
+  defp describe_error({:terrain_unavailable, {:gdal, _code, _output} = reason}) do
+    Logger.warning("CoverageWorker: GDAL failure: #{inspect(reason)}")
+    "Terrain data fetch failed. The TowerOps team has been notified — please retry shortly."
+  end
+
+  defp describe_error({:terrain_unavailable, reason}) do
+    Logger.warning("CoverageWorker: terrain fetch failed: #{inspect(reason)}")
+    "Terrain data is temporarily unavailable for this site. Please retry shortly."
+  end
 
   defp describe_error({:unknown_antenna, slug}), do: "Antenna #{inspect(slug)} is not in the registry."