feat(coverage): write rasters to NFS-backed dir, serve via dedicated static plug

Coverage rasters were being written to priv/static/coverage on the pod's
ephemeral filesystem — fine on a single-pod dev box, broken under k8s
where pods share NFS at /data and a restart wipes computed predictions.

* New :coverage_storage_dir config (defaults to priv/static/coverage for
  dev/test, "/data/coverage" in production).
* Raster.output_dir/absolute_raster_path read this config and resolve
  {otp_app, rel} or absolute paths.
* Endpoint now mounts a dedicated Plug.Static at /coverage from that
  same config — replicas all serve the same NFS-backed file regardless
  of which one ran the compute.
* Drop "coverage" from static_paths/0 since the dedicated plug owns
  that prefix now.

config/config.exs

@@ -123,6 +123,11 @@ config :towerops, ToweropsWeb.Endpoint,
   pubsub_server: Towerops.PubSub,
   live_view: [signing_salt: "Uh1ABfdI"]
 
+# Default coverage raster directory for dev/test. Production overrides this
+# to "/data/coverage" (the shared NFS mount) in config/prod.exs so all
+# replicas can serve the rasters and pod restarts don't lose them.
+config :towerops, :coverage_storage_dir, {:towerops, "priv/static/coverage"}
+
 # SNMP MIB directories for production (in Docker image)
 # MIB files are included in the release at /app/priv/mibs
 # MibTranslator automatically expands these to include subdirectories

config/prod.exs

@@ -39,3 +39,8 @@ config :swoosh, local: false
 # before starting your production server.
 config :towerops, ToweropsWeb.Endpoint, cache_static_manifest: "priv/static/cache_manifest.json"
 # SSL/TLS is handled by Cloudflared proxy - no force_ssl needed
+
+# Coverage rasters live on the shared NFS mount so all replicas can serve
+# them and a pod restart doesn't lose computed predictions. The
+# deployment mounts the NFS share at /data.
+config :towerops, :coverage_storage_dir, "/data/coverage"

lib/towerops/coverages/raster.ex

@@ -142,7 +142,15 @@ defmodule Towerops.Coverages.Raster do
   defp inside_bbox?(_coverage, _lat, _lon), do: false
 
   defp absolute_raster_path(%Coverage{raster_path: rel}) do
-    Path.join(static_dir(), Path.relative_to(rel, "/"))
+    # rel looks like "/coverage/<org>/<id>/rssi*.tif"; strip the URL prefix
+    # and rejoin against the configured storage dir (which already corresponds
+    # to /coverage in the static plug).
+    suffix =
+      rel
+      |> Path.relative_to("/")
+      |> String.replace_prefix("coverage/", "")
+
+    Path.join(coverage_dir(), suffix)
   end
 
   # gdallocationinfo with -wgs84 -valonly prints just the pixel value or
@@ -179,11 +187,19 @@ defmodule Towerops.Coverages.Raster do
   end
 
   defp output_dir(%Coverage{organization_id: org_id, id: id}) do
-    Path.join([static_dir(), "coverage", to_string(org_id), to_string(id)])
+    Path.join([coverage_dir(), to_string(org_id), to_string(id)])
   end
 
-  defp static_dir do
-    Application.app_dir(:towerops, "priv/static")
+  # Resolves :coverage_storage_dir config to an absolute path.
+  #
+  #   * `{:otp_app, "rel/path"}` → `Application.app_dir/2` (dev/test default
+  #     so rasters live under priv/static and Mix releases pick them up).
+  #   * `"/abs/path"` → used as-is (production points at the NFS mount).
+  defp coverage_dir do
+    case Application.get_env(:towerops, :coverage_storage_dir, {:towerops, "priv/static/coverage"}) do
+      {app, rel} when is_atom(app) and is_binary(rel) -> Application.app_dir(app, rel)
+      path when is_binary(path) -> path
+    end
   end
 
   defp relative_static_path(coverage, filename) do

lib/towerops_web.ex

@@ -17,7 +17,7 @@ defmodule ToweropsWeb do
   those modules here.
   """
 
-  def static_paths, do: ~w(assets fonts images vendor coverage favicon.ico robots.txt changelog.txt)
+  def static_paths, do: ~w(assets fonts images vendor favicon.ico robots.txt changelog.txt)
 
   def router do
     quote do

lib/towerops_web/endpoint.ex

@@ -41,6 +41,14 @@ defmodule ToweropsWeb.Endpoint do
     only: ToweropsWeb.static_paths(),
     raise_on_missing_only: code_reloading?
 
+  # Generated coverage rasters live outside priv/static so the path can point
+  # at a shared NFS mount in production (see :coverage_storage_dir). Keep this
+  # plug after the main static plug so /coverage URLs only fall through here.
+  plug Plug.Static,
+    at: "/coverage",
+    from: Application.compile_env(:towerops, :coverage_storage_dir, {:towerops, "priv/static/coverage"}),
+    gzip: false
+
   if Mix.env() == :dev do
     plug Tidewave
   end