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/common/configuration.nix

@@ -5,7 +5,7 @@
     [
       # Include the results of the hardware scan.
       #      ./hardware-configuration.nix
-      ../set-locale.nix
+      ./set-locale.nix
     ];
   # Use the GRUB 2 boot loader.
   # boot.loader.grub.enable = true;