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/__tests__/nodes.test.ts

@@ -63,7 +63,7 @@ describe('POST /api/nodes - api_url SSRF validation (C2 fix)', () => {
       .send({
         name: 'lan-node',
         type: 'remote',
-        api_url: 'http://192.168.1.50:3000',
+        api_url: 'http://192.168.1.50:1852',
         api_token: 'sometoken',
       });
     // Should succeed (201 or 200) - not a validation error