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.

backend/src/utils/validation.ts

@@ -22,7 +22,7 @@ export function isValidRemoteUrl(
     console.warn('[Validation] URL parse failure:', (e as Error).message, '— input:', raw);
     return {
       valid: false,
-      reason: 'API URL must be a valid URL (e.g. https://my-server.example.com:3000)',
+      reason: 'API URL must be a valid URL (e.g. https://my-server.example.com:1852)',
     };
   }
   if (!['http:', 'https:'].includes(url.protocol)) {