Add resiliency primitives: atomic writes, reconcile mutex, state backup

- Add storage.WriteFileAtomic (temp + rename) and storage.CopyFile helpers
- Convert all 8 production config writers to atomic writes: config/state.yaml,
  dnsmasq, nftables, domains, wireguard (config + secrets + peers + wg0.conf),
  tunnel/cloudflared
- Add sync.Mutex to Reconciler to serialize concurrent Reconcile() calls
  triggered by domain registration goroutines
- Add state.yaml backup (.bak) before every write; LoadState falls back to
  backup if primary is corrupted
- Reconciler refuses to use empty config on corruption (only on first run
  when no state file exists yet)

internal/tunnel/manager.go

@@ -19,6 +19,8 @@ import (
 	"strings"
 
 	"gopkg.in/yaml.v3"
+
+	"github.com/wild-cloud/wild-central/internal/storage"
 )
 
 // Config holds tunnel configuration.
@@ -143,12 +145,7 @@ func (m *Manager) WriteConfig(cfg Config, services []PublicDomain) error {
 		return nil
 	}
 
-	dir := filepath.Dir(m.configPath())
-	if err := os.MkdirAll(dir, 0755); err != nil {
-		return fmt.Errorf("creating tunnel config dir: %w", err)
-	}
-
-	if err := os.WriteFile(m.configPath(), []byte(content), 0644); err != nil {
+	if err := storage.WriteFileAtomic(m.configPath(), []byte(content), 0644); err != nil {
 		return fmt.Errorf("writing tunnel config: %w", err)
 	}