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/certbot/manager.go

@@ -61,6 +61,13 @@ func (m *Manager) Provision(domain, email string) error {
 	if email == "" {
 		return fmt.Errorf("email is required")
 	}
+	// Defense-in-depth: reject domains with shell metacharacters.
+	// Domain validation at the API boundary should already prevent this.
+	for _, c := range domain {
+		if !((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || (c >= '0' && c <= '9') || c == '-' || c == '.' || c == '*') {
+			return fmt.Errorf("invalid domain for cert provisioning: %q", domain)
+		}
+	}
 	if _, err := os.Stat(m.credsPath); err != nil {
 		return fmt.Errorf("cloudflare credentials not found at %s; configure the Cloudflare API token first", m.credsPath)
 	}