feat(course-package): add publish/unpublish with snapshot locking

- Add published_snapshot (jsonb) and published_at (timestamp) columns
- New POST publish/{id} endpoint: stores snapshot JSON, locks editing
- New POST unpublish/{id} endpoint: clears published_at for editing
- Block update and sync_items when published_at is set
- Frontend detail page with conditional edit/publish/unpublish buttons

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

htykc_models/migrations/2026-04-30-124521_add_published_snapshot_to_course_package/down.sql

@@ -0,0 +1,2 @@
+ALTER TABLE course_package DROP COLUMN IF EXISTS published_snapshot;
+ALTER TABLE course_package DROP COLUMN IF EXISTS published_at;

htykc_models/migrations/2026-04-30-124521_add_published_snapshot_to_course_package/up.sql

@@ -0,0 +1,2 @@
+ALTER TABLE course_package ADD COLUMN published_snapshot jsonb;
+ALTER TABLE course_package ADD COLUMN published_at timestamp;