Security hardening: input validation, secrets protection, NATS auth

Config injection prevention:
- Add FQDN validation for domain names (RFC 1123) in Register/Update —
  rejects newlines, spaces, shell metacharacters that could inject into
  HAProxy/dnsmasq configs
- Add backend address validation (valid host:port format, valid IP or
  hostname, port 1-65535). DNS-only backends allow bare IPs.
- Add header key/value validation — keys must be HTTP token chars only,
  values must not contain newlines or NULs
- Add WireGuard peer name validation (alphanumeric + hyphens + underscores)
- Add defense-in-depth domain validation in certbot Provision()

Secrets protection:
- Remove ?raw=true bypass on GET /api/v1/secrets — secrets are now always
  redacted in API responses regardless of query parameters
- Update test to verify redaction cannot be bypassed

NATS authentication:
- Generate random auth token on first startup, store in secrets.yaml
- Pass token to embedded NATS server via Authorization option
- Internal client connects with the same token
- External NATS clients (Wild Cloud) must now authenticate

Security headers:
- Add X-Content-Type-Options: nosniff
- Add X-Frame-Options: DENY
- Add Cache-Control: no-store

internal/api/v1/handlers_config_test.go

@@ -138,7 +138,7 @@ func TestGetGlobalSecrets_RedactsLeafValues(t *testing.T) {
 	}
 }
 
-func TestGetGlobalSecrets_RawShowsValues(t *testing.T) {
+func TestGetGlobalSecrets_AlwaysRedacted(t *testing.T) {
 	api, tmpDir := setupTestAPI(t)
 
 	secrets := map[string]any{
@@ -149,6 +149,7 @@ func TestGetGlobalSecrets_RawShowsValues(t *testing.T) {
 	data, _ := yaml.Marshal(secrets)
 	os.WriteFile(filepath.Join(tmpDir, "secrets.yaml"), data, 0600)
 
+	// Even with ?raw=true, secrets must be redacted (bypass removed for security)
 	req := httptest.NewRequest("GET", "/api/v1/secrets?raw=true", nil)
 	w := httptest.NewRecorder()
 
@@ -158,8 +159,8 @@ func TestGetGlobalSecrets_RawShowsValues(t *testing.T) {
 	json.Unmarshal(w.Body.Bytes(), &resp)
 
 	cf := resp["cloudflare"].(map[string]any)
-	if cf["apiToken"] != "my-token" {
-		t.Errorf("expected raw apiToken, got %v", cf["apiToken"])
+	if cf["apiToken"] == "my-token" {
+		t.Error("expected apiToken to be redacted, but got raw value")
 	}
 }