feat(van-weather): port weather service to Rust

Add a Rust implementation with privacy-masked locations, HTTP retries,
and Home Assistant sensor publishing. Package it with Nix and switch the
systemd service from Python to the compiled binary.

systems/brain/services/van_weather.nix

@@ -1,8 +1,10 @@
 {
   pkgs,
-  inputs,
   ...
 }:
+let
+  van-weather = pkgs.callPackage ../../../rust/van_weather/package.nix { };
+in
 {
   systemd.services.van-weather = {
     description = "Van Weather Service";
@@ -13,13 +15,9 @@
     requires = [ "home-assistant.service" ];
     wantedBy = [ "multi-user.target" ];
 
-    environment = {
-      PYTHONPATH = "${inputs.self}/";
-    };
-
     serviceConfig = {
       Type = "simple";
-      ExecStart = "${pkgs.my_python}/bin/python -m python.van_weather.main";
+      ExecStart = "${van-weather}/bin/van-weather";
       EnvironmentFile = "/etc/van_weather.env";
       Restart = "on-failure";
       RestartSec = "5s";
@@ -29,7 +27,6 @@
       ProtectSystem = "strict";
       ProtectHome = "read-only";
       PrivateTmp = true;
-      ReadOnlyPaths = [ "${inputs.self}" ];
     };
   };
 }