feat(auth): add API key authentication

Replace host port binding with expose so Coolify's Traefik handles
external routing. Remove PORT env var as it is no longer needed.

.env.example

@@ -1,2 +1,2 @@
 DOCLING_SERVE_ENABLE_UI=true
-PORT=5001
+DOCLING_SERVE_API_KEY=change-me

docker-compose.yml

@@ -1,10 +1,11 @@
 services:
   docling:
     image: quay.io/docling-project/docling-serve:latest
-    ports:
-      - "${PORT:-5001}:5001"
+    expose:
+      - "5001"
     environment:
       - DOCLING_SERVE_ENABLE_UI=${DOCLING_SERVE_ENABLE_UI:-true}
+      - DOCLING_SERVE_API_KEY=${DOCLING_SERVE_API_KEY}
     healthcheck:
       test: ["CMD", "curl", "-f", "http://localhost:5001/health"]
       interval: 30s