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.

docs/operations/self-hosting.mdx

@@ -70,7 +70,7 @@ Sencho itself is lightweight. The majority of resource usage on your host comes
 
 ## Networking
 
-- **Listen port:** 3000 (fixed). Map it to any host port using Docker's `-p` flag or `ports` in your compose file
+- **Listen port:** 1852 (fixed). Map it to any host port using Docker's `-p` flag or `ports` in your compose file
 - **Inbound:** Only the listen port needs to be reachable (directly or through a reverse proxy)
 - **Outbound:** No outbound connections are required for local-only setups. If you use multi-node management, Sencho needs HTTP/HTTPS access to remote Sencho instances on their configured API URLs
 - **Health check:** `GET /api/health` returns `200` when the application is ready. The Docker image includes a built-in `HEALTHCHECK` that polls this endpoint every 30 seconds

docs/operations/troubleshooting.mdx

@@ -557,7 +557,7 @@ docker compose pull && docker compose up -d
 Sencho exposes a health endpoint for monitoring and container health checks:
 
 ```bash
-curl http://localhost:3000/api/health
+curl http://localhost:1852/api/health
 # {"status":"ok","uptime":12345.67}
 ```