feat: add Pylon health relay for remote service health checks

DashCaddy Pylon is a lightweight probe agent that runs on remote
networks to relay health checks for services the main DashCaddy
instance can't reach directly (e.g., .sami domains, LAN IPs).

- Standalone zero-dependency Node.js script (pylon/dashcaddy-pylon.js)
- Optional API key auth, HEAD→GET fallback, batch probe support
- Health routes now try direct check first, fall back to pylon relay
- New endpoints: /health/probe (act as pylon), /health/pylon (status)
- Config: add "pylon": { "url": "...", "key": "..." } to config.json

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

dashcaddy-api/server.js

@@ -157,6 +157,7 @@ function loadSiteConfig() {
       siteConfig.configurationType = raw.configurationType || 'homelab';
       siteConfig.domain = raw.domain || '';
       siteConfig.routingMode = raw.routingMode || 'subdomain';
+      siteConfig.pylon = raw.pylon || null; // { url, key? } — health check relay
     }
   } catch (e) {
     // log.error may not be assigned yet during initial module load