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/traefik/rotate-logs.nix

@@ -0,0 +1,20 @@
+{ ... }:
+
+{
+      services.logrotate = {
+    enable = true;
+
+    settings = {
+      "/mnt/docker/volumes/traefik-data/logs/*.log" = {
+        daily = true;
+        size = "100M";
+        rotate = 20;
+        compress = true;
+        missingok = true;
+        notifempty = true;
+        copytruncate = true;
+      };
+
+     };
+  };
+}