refactor: extract duplicated patterns into shared helpers

Backend: create api/helpers.py with eligible_enrollment_filter,
count_eligible_students, get_active_enrollment, ensure_unique_name,
upsert_app_configs, and build_heatmap. Add full_name properties to
Student model. Apply across all route files (-481/+184 lines).

Frontend: create stores/helpers.js with withLoading composable,
apply to assessments and classes Pinia stores.

96/96 tests pass.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/api/routes/council.py

@@ -202,7 +202,7 @@ async def get_council_preparation(
             student_id=student.id,
             first_name=student.first_name,
             last_name=student.last_name,
-            full_name=f"{student.first_name} {student.last_name}",
+            full_name=student.full_name,
             overall_average=overall_average,
             assessment_count=assessment_count,
             grades_by_assessment=grades_by_assessment,