Reorganize loose modules into themed directories

Several single-purpose modules sat at modules/ root or in the
services/ catch-all despite the repo's established pattern of one
directory per concern (tailscale/, beszel/, docker/, nix-cache/):

- remote-builder-client.nix -> nix-cache/ (always co-included with
  nix-cache/client.nix in flake.nix's mkTarget, same buildType guard)
- set-locale.nix -> common/ (unconditionally imported by
  common/configuration.nix already)
- enable-ip-forwarding.nix -> networking/
- rotate-traefik-logs.nix -> traefik/rotate-logs.nix
- services/docker-health-to-gotify.nix and services/nextcloud-cron-job.nix
  -> docker/ (both only ever imported by the docker build type, same
  as the rest of modules/docker/*)

Pure path moves plus import-path updates in flake.nix,
common/configuration.nix, and build-types/docker.nix — verified
eval-equivalent (drvPath-identical) across representative hosts.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01La55Nsss8jZ7ZuzUV9mfot

modules/nix-cache/remote-builder-client.nix

@@ -0,0 +1,26 @@
+{ pkgs, vars, ... }:
+
+{
+  # Install the remote builder key on each client host (do not commit private keys):
+  #   sudo install -d -m 0700 /root/.ssh
+  #   sudo install -m 0600 ./nixremote /root/.ssh/nixremote
+  #   sudo ssh -i /root/.ssh/nixremote nixremote@nix-cache nix-store --version
+  nix.distributedBuilds = true;
+
+  nix.buildMachines = [
+    {
+      hostName = vars.nixCacheHost;
+      sshUser = vars.remoteBuilderUser;
+      sshKey = "/root/.ssh/${vars.remoteBuilderUser}";
+      system = pkgs.stdenv.hostPlatform.system;
+      maxJobs = 4;
+      speedFactor = 2;
+      supportedFeatures = [ "nixos-test" "benchmark" "big-parallel" "kvm" ];
+    }
+  ];
+
+  nix.settings = {
+    builders-use-substitutes = true;
+    max-jobs = "auto";
+  };
+}