refactor(core): introduce Unit of Work and remove commits from services

- Add UnitOfWork context manager in domain/unit_of_work.py with commit/rollback/flush API and auto-rollback on exception

- Remove all db.commit() from test_workflow_service (8 calls), notification_service (4 calls), status_service (1 call)

- Services now only stage changes via db.add/db.flush; caller owns the transaction boundary

- Update routers/tests.py: wrap 9 workflow endpoints in UnitOfWork context managers

- Update routers/notifications.py: wrap mark_as_read and mark_all_as_read in UnitOfWork

backend/app/routers/notifications.py

@@ -15,6 +15,7 @@ from sqlalchemy.orm import Session
 
 from app.database import get_db
 from app.dependencies.auth import get_current_user
+from app.domain.unit_of_work import UnitOfWork
 from app.models.notification import Notification
 from app.models.user import User
 from app.schemas.notification import NotificationOut, UnreadCountOut
@@ -78,12 +79,14 @@ def read_notification(
     current_user: User = Depends(get_current_user),
 ):
     """Mark a single notification as read."""
-    success = mark_as_read(db, notification_id, current_user.id)
-    if not success:
-        raise HTTPException(
-            status_code=status.HTTP_404_NOT_FOUND,
-            detail="Notification not found",
-        )
+    with UnitOfWork(db) as uow:
+        success = mark_as_read(db, notification_id, current_user.id)
+        if not success:
+            raise HTTPException(
+                status_code=status.HTTP_404_NOT_FOUND,
+                detail="Notification not found",
+            )
+        uow.commit()
     notif = db.query(Notification).filter(Notification.id == notification_id).first()
     return notif
 
@@ -99,5 +102,7 @@ def read_all_notifications(
     current_user: User = Depends(get_current_user),
 ):
     """Mark all notifications for the current user as read."""
-    count = mark_all_as_read(db, current_user.id)
+    with UnitOfWork(db) as uow:
+        count = mark_all_as_read(db, current_user.id)
+        uow.commit()
     return {"detail": f"Marked {count} notifications as read"}