feat: change default listen port from 3000 to 1852 (#756)

Updates the backend listen port, Vite dev proxy target, Docker EXPOSE,
compose port mapping, .env.example default, GitHub Actions smoke-test
default, healthcheck URLs, and every doc/example reference. Test fixtures
that include example URLs were updated for consistency, though their
assertions are port-agnostic.

The rate-limit value of 3000 in middleware/rateLimiters.ts and the
3000 entry in WEB_UI_PORTS (which detects user containers like Grafana)
are intentionally untouched.

Dockerfile

@@ -184,11 +184,11 @@ RUN sed -i 's/\r//' /usr/local/bin/docker-entrypoint.sh \
   && chmod +x /usr/local/bin/docker-entrypoint.sh
 
 # Expose port
-EXPOSE 3000
+EXPOSE 1852
 
 # Health check - polls the public /api/health endpoint every 30s
 HEALTHCHECK --interval=30s --timeout=5s --start-period=15s --retries=3 \
-  CMD node -e "const h=require('http');h.get('http://localhost:3000/api/health',r=>{process.exit(r.statusCode===200?0:1)}).on('error',()=>process.exit(1))"
+  CMD node -e "const h=require('http');h.get('http://localhost:1852/api/health',r=>{process.exit(r.statusCode===200?0:1)}).on('error',()=>process.exit(1))"
 
 # Entrypoint ensures /app/data is writable and execs the CMD as root by default,
 # or drops to $SENCHO_USER via su-exec when that env var is set (see comment above).