feat: Self-register Wild Central on startup

On startup, Wild Central registers its own domain (from config
cloud.central.domain) as an HTTP service with itself. This creates
the DNS entry, proxy route, and TLS cert needed to access the
Central UI via its domain name.

Registration is idempotent — safe on every restart. Skips silently
if no domain is configured. Logs a warning if registration fails.

Verified: after startup, /api/v1/services shows wild-central
registered with domain central.payne.io.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

main.go

@@ -150,6 +150,9 @@ func main() {
 		fmt.Sscanf(v, "%d", &port)
 	}
 
+	// Self-register Wild Central's own domain (idempotent, skips if no domain configured)
+	api.RegisterSelf(port)
+
 	addr := fmt.Sprintf("%s:%d", host, port)
 	slog.Info("wild-central started", "addr", addr, "version", Version)