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/classes.py

@@ -12,6 +12,7 @@ from sqlalchemy import select, func
 from sqlalchemy.orm import selectinload
 
 from api.dependencies import AsyncSessionDep
+from api.helpers import ensure_unique_name
 from infrastructure.database.models import (
     ClassGroup,
     Student,
@@ -201,7 +202,7 @@ async def get_class_students(
                 last_name=student.last_name,
                 first_name=student.first_name,
                 email=student.email,
-                full_name=f"{student.first_name} {student.last_name}",
+                full_name=student.full_name,
                 current_class_id=class_id if is_active else None,
                 current_class_name=cls.name if is_active else None,
                 enrollment_id=enrollment.id,
@@ -419,13 +420,7 @@ async def create_class(
     Crée une nouvelle classe.
     """
     # Vérifier l'unicité du nom
-    existing_query = select(ClassGroup).where(ClassGroup.name == class_data.name)
-    existing_result = await session.execute(existing_query)
-    if existing_result.scalar_one_or_none():
-        raise HTTPException(
-            status_code=400, 
-            detail=f"Une classe avec le nom '{class_data.name}' existe déjà"
-        )
+    await ensure_unique_name(session, ClassGroup, class_data.name, entity_label="classe")
     
     # Créer la nouvelle classe
     new_class = ClassGroup(
@@ -466,16 +461,10 @@ async def update_class(
     
     # Vérifier l'unicité du nouveau nom si modifié
     if class_data.name and class_data.name != cls.name:
-        existing_query = select(ClassGroup).where(
-            ClassGroup.name == class_data.name,
-            ClassGroup.id != class_id
+        await ensure_unique_name(
+            session, ClassGroup, class_data.name,
+            exclude_id=class_id, entity_label="classe"
         )
-        existing_result = await session.execute(existing_query)
-        if existing_result.scalar_one_or_none():
-            raise HTTPException(
-                status_code=400, 
-                detail=f"Une autre classe avec le nom '{class_data.name}' existe déjà"
-            )
     
     # Appliquer les modifications
     if class_data.name is not None: