Update README, fix async user management, and optimize config loading

[Cylae]

• Updated README.md and Cargo.toml to version 1.0.8.
• Implemented add_user_async and delete_user_async in UserManager to offload blocking I/O and CPU-intensive tasks to spawn_blocking.
• Updated web.rs handlers to use the new async user management methods.
• Optimized Config::load_async with a 500ms throttle on file metadata checks.
• Added enable_service_async and disable_service_async to Config for safe, non-blocking configuration updates.

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PR created automatically by Jules for task 10550500890751589869 started by @Cylae

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