Skip redundant library sync when other user has access

[Majorfi]

Summary

Optimize the library sync process by skipping redundant syncs when another user already has access to the same library. This reduces unnecessary database operations and improves sync efficiency in multi-user scenarios.

What changed

• Added database method to check if another user has access to a library
• Updated sync service logic to skip syncing when access already exists via another user
• Extended interfaces to support the new access check
• Added comprehensive unit tests for both the database check and sync service behavior (50+ test lines)

Risks

• Verify access checks work correctly across different permission scenarios
• Test sync behavior with shared libraries and concurrent user access
• Monitor edge cases where permissions change during sync operations
• Ensure the optimization doesn't hide legitimate sync needs

Test plan

• Run full test suite to verify no regressions
• Test sync with multiple users accessing the same library
• Verify sync is correctly skipped when another user has access
• Verify sync proceeds normally when only the current user has access
• Performance test with large libraries and multiple users

Docs impact

None

Breaking changes

None