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/middleware.go

@@ -49,6 +49,11 @@ func RequestLoggingMiddleware(next http.Handler) http.Handler {
 			return
 		}
 
+		// Security response headers
+		w.Header().Set("X-Content-Type-Options", "nosniff")
+		w.Header().Set("X-Frame-Options", "DENY")
+		w.Header().Set("Cache-Control", "no-store")
+
 		start := time.Now()
 		sw := &statusResponseWriter{ResponseWriter: w, status: http.StatusOK}
 		next.ServeHTTP(sw, r)