Restore the green release matrix: Course creation hits NOT NULL on courses_course.archive_commit_sha

Plan context: CI run 34189057638 (87fd3f4f) is the first completed django verdict on main since Sep 5 (earlier runs were cancelled by the container/quality failures fixed in #337/#319/#341/#343): quality, container, playwright and screenshots now pass, but the django job fails and ci-gate reports failure. Dozens of tests ERROR at setup with sqlite3.IntegrityError: NOT NULL constraint failed: courses_course.archive_commit_sha. Introduced by the shared-curriculum merge (#320).

Symptom

• Failing modules include courses.tests.test_course_list_ordering, courses.tests.test_course_list_metadata, courses.tests.test_course_list_assignments, courses.tests.test_homework_optional_fields, courses.tests.test_local_question_seed, courses.tests.test_local_project_review_seed, studio_courses.tests (course/homework/project views), core.tests.test_course_platform_vendor_assets, review_import.tests.test_workflow.
• The model field (courses/models/cohort.py:192) declares default="", and the visible 0005 AddField declares default="" too, yet some INSERT into courses_course omits the column entirely.
• The visible AddField in courses/migrations/0005_shared_current_curriculum.py names model cohort, while the failing table is courses_course; a missing or default-less AddField for the course model is the prime suspect.

Direction

• Inventory every archive field AddField in migration 0005 and which model each targets; run the migration-drift check.
• Identify the creation path that bypasses the model default (missing AddField, bulk/raw insert, or fixture) and fix it at the root per the model's documented semantics: empty string for current cohorts, full 40-hex commit SHA for github_archive cohorts; the archive identity check constraint stays intact.

Scope

• The smallest change that makes Course creation safe everywhere the model default was assumed. No product behavior changes beyond what the model already specifies. No workflow or quality-contract edits.

Acceptance criteria

• The migration-drift check passes.
• All modules listed above pass locally.
• The full Django suite passes locally.
• Post-merge CI on main: the django job passes and the run reaches a green ci-gate verdict with no cancellations.

Start with courses/models/cohort.py:192 and courses/migrations/0005_shared_current_curriculum.py, inventorying every archive field AddField and its target model. Run the migration-drift check, then trace the Course creation path that omits archive_commit_sha, including the listed failing modules. Done means the drift check, listed modules, full Django suite, and post-merge django CI gate all pass.