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/infrastructure/database/models.py

@@ -123,6 +123,16 @@ class Student(Base):
         "CouncilAppreciation", back_populates="student", lazy="selectin"
     )
 
+    @property
+    def full_name(self) -> str:
+        """Prénom Nom"""
+        return f"{self.first_name} {self.last_name}"
+
+    @property
+    def full_name_reversed(self) -> str:
+        """Nom Prénom"""
+        return f"{self.last_name} {self.first_name}"
+
     def __repr__(self):
         return f"<Student {self.first_name} {self.last_name}>"