Add HTTP resync endpoint for manual rotation triggering

Expose port 8080 with `/resync` endpoint to trigger on-demand WireGuard rotations. Add RESYNC_PORT and RESYNC_BIND environment variables. Implement thread-safe rotation locking to prevent concurrent runs. Update README with endpoint usage examples.

Dockers/gluetun-pia-wireguard-rotator/Dockerfile

@@ -26,5 +26,8 @@ ENV PIA_CA_PATH=/config/cache/ca.rsa.4096.crt
 ENV LATENCY_SWITCH_MARGIN_MS=15
 ENV HEALTH_CHECK_INTERVAL=10
 ENV UNHEALTHY_ROTATE_COOLDOWN=60
+ENV RESYNC_PORT=8080
+
+EXPOSE 8080
 
 ENTRYPOINT ["/usr/local/bin/entrypoint.sh"]