fix(rover): bound Overpass with hard 8s deadline + 6h ETS cache by bbox

Prod logs showed road_proximity took 186 s on a single Calculate.
Req's receive_timeout only catches TCP idle, not slow streaming, so a
trickling Overpass response could not be cancelled. Wrap the call in a
Task.yield/shutdown deadline and cache successful results by 0.05°-rounded
bbox so adjacent Calculates skip Overpass entirely.

lib/microwaveprop/rover/road_proximity.ex

@@ -12,12 +12,16 @@ defmodule Microwaveprop.Rover.RoadProximity do
   `:unavailable` and the scorer falls back to ignoring road proximity.
   """
 
+  alias Microwaveprop.Cache
+
   require Logger
 
   @overpass_url "https://overpass-api.de/api/interpreter"
   @road_types ~w(motorway trunk primary secondary tertiary unclassified residential)
-  @overpass_query_timeout_s 10
-  @overpass_receive_timeout_ms 12_000
+  @overpass_query_timeout_s 6
+  @overpass_receive_timeout_ms 8_000
+  @overpass_hard_deadline_ms 8_000
+  @cache_ttl_ms 6 * 60 * 60 * 1_000
 
   @type latlon :: {float(), float()}
   @type segment :: {latlon(), latlon()}
@@ -42,7 +46,21 @@ defmodule Microwaveprop.Rover.RoadProximity do
   end
 
   @spec fetch_segments(bbox()) :: {:ok, [segment()]} | {:error, term()}
-  def fetch_segments(%{"south" => s, "west" => w, "north" => n, "east" => e}) do
+  def fetch_segments(%{"south" => _, "west" => _, "north" => _, "east" => _} = bbox) do
+    key = {:rover_road_segments, cache_key(bbox)}
+
+    case Cache.fetch_or_store(key, @cache_ttl_ms, fn -> do_fetch_segments(bbox) end) do
+      {:ok, _segments} = ok ->
+        ok
+
+      {:error, _} = err ->
+        # Don't poison the cache with errors — subsequent Calculates retry.
+        Cache.invalidate(key)
+        err
+    end
+  end
+
+  defp do_fetch_segments(%{"south" => s, "west" => w, "north" => n, "east" => e}) do
     types = Enum.join(@road_types, "|")
 
     query = """
@@ -51,22 +69,37 @@ defmodule Microwaveprop.Rover.RoadProximity do
     out geom;
     """
 
-    case Req.post(@overpass_url,
-           form: [data: query],
-           receive_timeout: @overpass_receive_timeout_ms,
-           retry: false
-         ) do
-      {:ok, %{status: 200, body: %{"elements" => elements}}} ->
+    task =
+      Task.async(fn ->
+        Req.post(@overpass_url,
+          form: [data: query],
+          receive_timeout: @overpass_receive_timeout_ms,
+          retry: false
+        )
+      end)
+
+    case Task.yield(task, @overpass_hard_deadline_ms) || Task.shutdown(task, :brutal_kill) do
+      {:ok, {:ok, %{status: 200, body: %{"elements" => elements}}}} ->
         {:ok, segments_from_elements(elements)}
 
-      {:ok, %{status: status}} ->
+      {:ok, {:ok, %{status: status}}} ->
         {:error, {:http, status}}
 
-      {:error, reason} ->
+      {:ok, {:error, reason}} ->
         {:error, reason}
+
+      nil ->
+        {:error, :overpass_deadline_exceeded}
     end
   end
 
+  # Round to 0.05° (~5 km) so adjacent Calculate calls share cache hits.
+  defp cache_key(%{"south" => s, "west" => w, "north" => n, "east" => e}) do
+    {round_to(s, 0.05), round_to(w, 0.05), round_to(n, 0.05), round_to(e, 0.05)}
+  end
+
+  defp round_to(v, step), do: Float.round(v / step) * step
+
   defp segments_from_elements(elements) do
     Enum.flat_map(elements, fn
       %{"type" => "way", "geometry" => geom} when is_list(geom) and length(geom) >= 2 ->