fix: staff only commands

[Akinator31]

This pull request tightens command access controls to ensure that both slash and text commands are strictly limited to staff members, with precise logic for dual-server and single-server modes. It introduces utility functions to centralize staff-checking logic, updates the command handlers to enforce staff-only access, and revises the documentation to clarify the new permissions model.

Access Control Enforcement:

• Slash command handling in guild_interaction_handler.rs now checks if the user is staff (using the new is_staff_member and is_staff_user helpers) before allowing command execution. Non-staff users receive a permission denied response.
• Text command handling in guild_messages_handler.rs now silently ignores commands from non-staff users.

Utility Functions:

• Added is_staff_member and is_staff_user helpers in new module utils/command/staff_access.rs to encapsulate staff-checking logic for both dual and single server modes, including super admin handling.
• Re-exported the new helpers in utils/command/mod.rs for easier access throughout the codebase.
• Updated imports in guild_interaction_handler.rs to use the new helpers.

Documentation:

• Updated docs/guides/commands.md to clearly explain the new staff-only command restrictions and how they differ between dual-server and single-server modes.