fix: preserve existing settings during default init

[imleoo]

Summary

• backfill missing default settings instead of using a single sentinel key
• preserve existing database-backed settings during startup/default initialization
• add unit coverage for partial settings backfills and existing value preservation

Tests

• go test -tags=unit ./internal/service -run 'TestSettingService_InitializeDefaultSettings'
• go test -tags=unit ./internal/service
• git diff --check

[github-actions]

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