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.go

@@ -406,11 +406,7 @@ func (api *API) GetGlobalSecrets(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	showRaw := r.URL.Query().Get("raw") == "true"
-	if !showRaw {
-		redactSecrets(secretsMap)
-	}
-
+	redactSecrets(secretsMap)
 	respondJSON(w, http.StatusOK, secretsMap)
 }