Add flexible Flask port configuration via environment variable

- Add FLASK_PORT environment variable support in docker-compose.yml
- Update Flask app to read port from FLASK_PORT environment variable
- Add FLASK_PORT configuration to .env and .env.example files
- Enables running Flask on custom ports to avoid port conflicts in containerized environments

🤖 Generated with [Claude Code](https://claude.ai/code)

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

.env.example

@@ -10,6 +10,9 @@ WOL_NETWORK_MODE=bridge
 # For Linux with host mode: this setting is ignored
 WOL_EXTERNAL_PORT=5000
 
+# Flask internal port (port on which Flask app runs inside container)
+FLASK_PORT=5000
+
 # Examples:
 # Windows Docker Desktop:
 # WOL_NETWORK_MODE=bridge