feat: +thin-client role + AZ-TC-01..03 pilot

Auf Basis des Grilling-Sessions mit 20 design decisions umgesetztes
v1-Skeleton der Thin-Client-Rolle plus 3 Pilot-Hosts.

Architektur:
- roles/thin-client/ als geschlossene Rolle (default.nix compose + 7
  Subdirectories: hardware, session, identity, network, peripherals,
  apps, monitoring, deployment)
- hosts/AZ-TC-NN/default.nix als ~20-Zeilen-Wrapper pro Fleet-Host
- flake.nix instanziiert AZ-TC-01..03 via Fleet-Helper
- secrets.nix mit per-host agenix-Secret-Stubs

Submodule:
- hardware: dell-optiplex-micro + generic-x86_64-uefi Fallback
- identity: AD (sssd/krb5/keytab via agenix), lokale Notfalluser
  (sascha.koenig + jannik.mueller ohne m3ta-home), sudo-Policy
- session: KDE Plasma 6 + Wayland + SDDM, Branding (Wallpaper + Footer),
  PipeWire Audio
- network: NetworkManager + wpa_supplicant + 802.1X EAP-TLS, NetBird
  + SSH via NetBird, systemd-resolved (Corp + NetBird DNS), hardened
  firewall, OpenSSH
- peripherals: CUPS mit Pull-Print-Queue, pam_mount für DFS-Shares
- apps: Chromium (ManagedBookmarks, Bitwarden force-install, no local
  passwords), Office-Web .desktop-Shortcuts, Remmina (mehrere TS,
  Kerberos SSO), RustDesk Client + Daemon, OBS Studio, Autostart
- monitoring: node_exporter → Pushgateway, Alloy (stub für Loki),
  Snipe-IT Asset-Checkin (stub)
- deployment: Disko BTRFS-Layout, auto-upgrade daily + reboot window,
  snapper snapshots

Build-Validierung: 'nix flake check' bestanden für AZ-TC-01/02/03.

Siehe roles/thin-client/README.md für den Provisionierungs-Workflow
und die Liste der noch auszufüllenden Platzhalter (TODO-Kommentare
in den jeweiligen Modulen).

roles/thin-client/network/dns.nix

@@ -0,0 +1,24 @@
+# roles/thin-client/network/dns.nix
+#
+# DNS strategy: Corp DNS (AD DCs) as primary, NetBird DNS as overlay
+# for NetBird-managed domains via systemd-resolved.
+#
+# Q15 decision: "Corp DNS + NetBird DNS parallel".
+{config, lib, ...}: let
+  inherit (lib) mkIf;
+  cfg = config.az.tc;
+in {
+  config = mkIf cfg.enable {
+    services.resolved = {
+      enable = true;
+      settings.Resolve = {
+        Domains = ["az-group.local"];
+        FallbackDNS = ["9.9.9.9" "1.1.1.1"];
+        LLMNR = "no";
+        MulticastDNS = "no";
+      };
+    };
+
+    # NetBird hooks into resolved via its own daemon (no extra config here).
+  };
+}