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/networking/enable-ip-forwarding.nix

@@ -0,0 +1,12 @@
+{ config, lib, ... }:
+
+{
+  options.myNetworking.ipForwarding.enable = lib.mkEnableOption "Enable IPv4 and IPv6 forwarding";
+
+  config = lib.mkIf config.myNetworking.ipForwarding.enable {
+    boot.kernel.sysctl = {
+      "net.ipv4.ip_forward" = 1;
+      "net.ipv6.conf.all.forwarding" = 1;
+    };
+  };
+}