Promote sessions.repository_id to a real foreign key

[bme2003]

Summary

sessions.repository_id is currently stored as a plain string. It should be a true foreign key to repositories.id.

Why

This improves data integrity and makes it easier to build richer queries safely.

Proposed Change

• Update SQLAlchemy model
• Add Alembic migration
• Verify existing API behavior still works

Acceptance Criteria

• sessions.repository_id references repositories.id
• Migration is included
• Tests pass after migration

[IlyasFardaouix]

I think the root cause of the issue is that the sessions table was created before the repositories table, which means the foreign key constraint can't be properly established. To fix this, we should reorder the migration scripts so that 0002_add_repositories is applied before 0001_initial_schema. Does that sound about right?