Change Hub port from 3000 to 3847

- Add HUB_PORT variable to .env.example
- Update docker-compose.yml to use configurable port
- Update nginx config to proxy to port 3847

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

deploy/.env.example

@@ -28,6 +28,9 @@ SETTINGS_ENCRYPTION_KEY=CHANGE_ME_GENERATE_WITH_OPENSSL_RAND_HEX_32
 POSTGRES_USER=letsbe_hub
 POSTGRES_DB=letsbe_hub
 
+# Hub port (change if 3000 is occupied)
+HUB_PORT=3847
+
 # Hub image tag (default: master)
 HUB_IMAGE_TAG=master