feat(backend): make backend configurable via environment variables

[alikhere]

Summary

Every configurable value in the Flask backend was hardcoded in main.py— host, port, debug mode, CORS origins, and output directory path. None of it could be changed without modifying source code directly.

Wired all five values to environment variables using os.environ.get() with the existing hardcoded values as defaults, so behaviour is completely unchanged when no .env is present:

• FLASK_HOST / FLASK_PORT / FLASK_DEBUG → app.run()
• CORS_ORIGINS → CORS(app, origins=...)
• OUTPUT_DIR → module-level variable used in start_gdb_session,
  compile_code, and upload_file

Also added gdbui_server/.env.example documenting all five variables, and a short Environment Variables section in README.md.

How to test

• Copy the example and start the server — should behave exactly as before:
  cp gdbui_server/.env.example gdbui_server/.env && python main.py
• Set FLASK_PORT=9999 in .env and restart — server should bind to 9999
• Confirm tests still pass:
  cd gdbui_server && python -m unittest discover -s . -p 'flask_test.py'

Related Issue

Fixes #185