[HnR-Autograder:1] DB migration

[Elimpizza]

This pull request introduces a database schema migration to support tracking the predecessor of each auto grading configuration for assignments. The main change is the addition of a self-referential foreign key to the assignment_auto_grading_config table, allowing each configuration to reference its previous version. This enables a linear history of configuration changes without allowing forks.

Schema changes to assignment_auto_grading_config:

• Added a nullable previous_config_id column to assignment_auto_grading_config, allowing each configuration to reference a predecessor.
• Created a foreign key constraint on previous_config_id referencing the id of the same table, with cascading deletes.
• Added a unique constraint on previous_config_id to ensure that each configuration can only be the predecessor of at most one other configuration, preventing forks in the configuration history.