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/src/components/NodeManager.tsx

@@ -368,7 +368,7 @@ export function NodeManager() {
             <Label htmlFor="node-api-url">Sencho API URL</Label>
             <Input
               id="node-api-url"
-              placeholder="http://192.168.1.50:3000"
+              placeholder="http://192.168.1.50:1852"
               value={formData.api_url}
               onChange={(e) => setFormData({ ...formData, api_url: e.target.value })}
             />