Reorganize services/ into category subfolders

Group service modules by category (media, network, vpn, identity,
dev, desktop) to make the growing services/ dir easier to navigate.
containers.nix stays at the top level since it's a shared backend,
not a single-category service.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

hosts/mercury/configuration.nix

@@ -7,8 +7,8 @@
   imports = [
     ../../common.nix           # shared base: user / ssh / nix / firewall
     ./secrets.nix              # sops-nix: darman password (age key on boot part.)
-    ../../services/unbound.nix      # local recursive resolver (127.0.0.1:5335)
-    ../../services/pihole.nix        # DNS adblock + DHCP (declarative static leases)
+    ../../services/network/unbound.nix      # local recursive resolver (127.0.0.1:5335)
+    ../../services/network/pihole.nix        # DNS adblock + DHCP (declarative static leases)
   ];
 
   networking.hostName = "mercury";