Add semester field to courses

[alxstx]

Changes

• Added database migration to add semester string column to the courses table
• Course model populates semester from Canvas term data (e.g. "Spring 2026") when importing courses
• Added by_semester scope to Course model for filtering courses by semester
• Added backfill rake task (rake courses:backfill_semester) to populate semester for existing courses by fetching term data from Canvas API

Testing

• Run bundle exec rspec to verify all tests pass
• After deploying, run the migration and backfill task:

  heroku run rails db:migrate -a sp26-02-flextensions
  heroku run rake courses:backfill_semester -a sp26-02-flextensions
  

Documentation

No documentation needed

Checklist

• Name of branch corresponds to story
• 80%+ test coverage with all tests passing