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__/template-service.test.ts

@@ -210,11 +210,11 @@ describe('TemplateService', () => {
     it('handles values with special characters', () => {
       const result = service.generateEnvString({
         PASSWORD: 'p@ss=word!',
-        URL: 'http://localhost:3000',
+        URL: 'http://localhost:1852',
       });
 
       expect(result).toContain('PASSWORD=p@ss=word!');
-      expect(result).toContain('URL=http://localhost:3000');
+      expect(result).toContain('URL=http://localhost:1852');
     });
   });