feat: Implement ON DELETE CASCADE for sequencer_projects foreign key

Commit notes

- Added migration to ensure the pack_id foreign key on sequencer_projects references sample_packs(id) with ON DELETE CASCADE, preventing orphaned records. - Updated DeleteSamplePack handler to delete associated sequencer projects when a sample pack is deleted, ensuring referential integrity. - Modified SequencerProject model to enforce cascading delete behavior on the foreign key constraint.