docs(arch): update settingsservices md

[leeeunkoo]

📝 PR Description

This PR includes documentation updates for the SettingsService in the PICCOLO framework. Key changes include:

1. Improved structure of the SettingsService LLD (Low Level Design) document
2. Added 'settings_' prefix to module names for naming convention consistency
3. Simplified module structure using mod.rs files
4. Added missing sections: external system integration, monitoring, deployment strategies, and security considerations
5. Ensured consistency between KR/EN documents

This update resolves the gaps between HLD and LLD documents, providing developers with comprehensive information needed for SettingsService implementation.

🔗 Related Issue

related #174

🧪 Test Method

1. Compared all sections between HLD and LLD documents to ensure no content is missing
2. Reviewed English and Korean versions for content consistency
3. Technically reviewed the proposed file structure for implementation suitability
4. Verified consistency of module naming conventions

📸 Screenshots

N/A (Document update with no UI changes)

✅ Checklist

• Code conventions are followed
• Tests are added/modified
• Documentation is updated (if necessary)
• Consistency between HLD and LLD documents is ensured
• Content consistency between KR/EN documents is maintained
• All essential sections are included (configuration, security, deployment, external system integration, etc.)