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/deployment/snapper.nix

@@ -0,0 +1,40 @@
+# roles/thin-client/deployment/snapper.nix
+#
+# BTRFS snapshots for the root subvolume via snapper. Snapshots taken
+# before and after each `nixos-rebuild switch`, plus hourly/daily.
+#
+# Q18 decision: Generation-Picker + BTRFS-Snapshot.
+{config, lib, ...}: let
+  inherit (lib) mkIf;
+  cfg = config.az.tc;
+in {
+  config = mkIf cfg.enable {
+    services.snapper = {
+      snapshotRootOnBoot = true;
+      configs = {
+        root = {
+          SUBVOLUME = "/";
+          ALLOW_USERS = ["root"];
+          TIMELINE_CREATE = true;
+          TIMELINE_CLEANUP = true;
+          TIMELINE_LIMIT_HOURLY = "12";
+          TIMELINE_LIMIT_DAILY = "7";
+          TIMELINE_LIMIT_WEEKLY = "4";
+          TIMELINE_LIMIT_MONTHLY = "0";
+        };
+      };
+    };
+
+    # Take a snapshot before any nixos-rebuild switch — hooks into the
+    # toplevel system path activation.
+    system.activationScripts.snapperPreSwitch = {
+      deps = [];
+      text = ''
+        if [ -d /.snapshots ]; then
+          ${config.services.snapper.package}/bin/snapper -c root create \
+            -d "pre-switch $(date +%Y-%m-%d-%H%M)" || true
+        fi
+      '';
+    };
+  };
+}