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.

.github/actions/start-app/action.yml

@@ -13,7 +13,7 @@ inputs:
     default: '/tmp/compose'
   port:
     required: false
-    default: '3000'
+    default: '1852'
   skip-backend-build:
     description: >
       When 'true', skip running `npm run build` in backend/ and assume the

.github/workflows/docker-publish.yml

@@ -126,12 +126,12 @@ jobs:
           # enough for `docker logs` in the trap to surface the stack trace.
           # The trap force-removes it explicitly whether it is still running
           # or already exited.
-          docker run -d --name sencho-smoke -p 3000:3000 localhost/sencho:release-scan
+          docker run -d --name sencho-smoke -p 1852:1852 localhost/sencho:release-scan
           trap 'docker logs sencho-smoke 2>&1 || true; docker rm -f sencho-smoke >/dev/null 2>&1 || true' EXIT
           for i in $(seq 1 30); do
-            if curl -fsS http://localhost:3000/api/health >/dev/null 2>&1; then
+            if curl -fsS http://localhost:1852/api/health >/dev/null 2>&1; then
               echo "Container healthy after ${i}s"
-              curl -s http://localhost:3000/api/health
+              curl -s http://localhost:1852/api/health
               exit 0
             fi
             sleep 1