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.

.env.example

@@ -7,8 +7,8 @@ JWT_SECRET=your-secure-jwt-secret-here
 # Directory containing docker-compose files
 COMPOSE_DIR=/path/to/your/compose/files
 
-# HTTP server port (default: 3000)
-PORT=3000
+# HTTP server port (default: 1852)
+PORT=1852
 
 # Database and state directory inside the container (default: /app/data)
 DATA_DIR=/app/data