compose: make host port configurable (APP_PORT)

Host port 3000 can clash with other services on the deploy host; allow
overriding via APP_PORT (default 3000). Only needed for Traefik to reach
the app — can be dropped entirely if Traefik shares the Docker network.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docker-compose.yml

@@ -34,8 +34,11 @@ services:
       - uploads:/app/uploads
       - thumbnails:/app/thumbnails
       - glb:/app/glb
+    # Host port is only for Traefik to reach the app; set APP_PORT to any free
+    # port (3000 was already taken on the host). If Traefik shares the Docker
+    # network and routes by container, you can drop this whole `ports:` block.
     ports:
-      - "3000:3000"
+      - "${APP_PORT:-3000}:3000"
     restart: unless-stopped
     # Traefik (if using labels rather than your file provider):
     # labels: