Add error handling for Q&A trees preload failure

[Copilot]

Addresses feedback from PR #5: wrap preload_directory() in error handling that logs and exits on failure.

Changes

• Wrap qna_service.preload_directory() in try-except
• Log error and call sys.exit(1) on failure to prevent startup in broken state

try:
    await qna_service.preload_directory(config_dir)
except Exception as e:
    logger.error(f"Failed to preload Q&A trees: {e}")
    sys.exit(1)

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