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.

docker-compose.yml

@@ -5,7 +5,7 @@ services:
     container_name: sencho
     restart: unless-stopped
     ports:
-      - "3000:3000"
+      - "1852:1852"
     volumes:
       # Required: Docker Socket for container orchestration
       - /var/run/docker.sock:/var/run/docker.sock