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.

frontend/vite.config.ts

@@ -27,12 +27,12 @@ export default defineConfig({
   server: {
     proxy: {
       '/api': {
-        target: 'http://localhost:3000',
+        target: 'http://localhost:1852',
         changeOrigin: true,
         ws: true,
       },
       '/ws': {
-        target: 'http://localhost:3000',
+        target: 'http://localhost:1852',
         changeOrigin: true,
         ws: true,
       },