Implement role and user management enhancements

- Added MaxIpLimit to roles, allowing for the configuration of simultaneous IP limits for users.
- Updated role creation and update commands to include MaxIpLimit, ensuring proper handling in the application logic.
- Enhanced user management by introducing a DELETE endpoint for user accounts, with appropriate checks to prevent self-deletion.
- Updated documentation to reflect changes in role and user management, clarifying the new IP limit functionality and user deletion process.
- Adjusted related tests to cover new functionality and ensure robust validation of role and user management features.

backend/src/PnvPanel.Infrastructure/Persistence/Migrations/AppDbContextModelSnapshot.cs

@@ -535,6 +535,9 @@ namespace PnvPanel.Infrastructure.Persistence.Migrations
                     b.Property<int>("MaxConfigs")
                         .HasColumnType("integer");
 
+                    b.Property<int>("MaxIpLimit")
+                        .HasColumnType("integer");
+
                     b.Property<string>("Name")
                         .HasMaxLength(256)
                         .HasColumnType("character varying(256)");