feat: pridat netbird roli do priprav-novy-server.yml

Nove role:
- roles/netbird/ - instaluje NetBird mesh VPN klient

Konfigurace:
- netbird_setup_key: '' (placeholder, dopln v group_vars/all.yml)
- netbird_run_as_system_service: true
- netbird_management_url: 'https://api.netbird.io'

Playbook priprav-novy-server.yml:
- pridana role netbird mezi docker a fail2ban
- update hlavicky (krok 8 NetBird)
- update README strom (netbird role)

roles/netbird/defaults/main.yml

@@ -0,0 +1,15 @@
+---
+# Promenne pro netbird roli
+# NetBird - WireGuard-based mesh VPN (https://netbird.io)
+
+# Setup klice pro NetBird registraci
+# Najdes v NetBird dashboardu: https://app.netbird.io/ -> Setup Keys
+# Napr: AAAA-BBBB-CCCC-DDDD-EEEE
+netbird_setup_key: ""
+
+# NetBird spust jako systemova sluzba (root)
+# Pro user-level: netbird up --setup-key XXX (interaktivni)
+netbird_run_as_system_service: true
+
+# Vychozi Management Server (self-hosted nebo oficialni)
+netbird_management_url: "https://api.netbird.io"