Remove the Begin of the insertionlegacy `Sprint` model that still subclasses `Version`, together with the remaining code paths that depend on version-backed sprint behaviour.

Once theEnd of the insertion old Begin of the insertionmodel and its compatibility layer have been removed, rename `Agile::Sprint` to `Sprint` so thatEnd of the insertion Begin of the deletion Sprint model, then removeEnd of the deletion the Begin of the insertiondedicated sprint model becomes the canonical implementation.

This ticket should focus on the model boundary and remaining dependencies on the legacy sprint implementation. _Route consolidation, controller naming cleanup, and other Backlogs refactorings should remain in their respective child tickets._

### **Definition of done**

* The legacy `Sprint < Version` model has been removed.

* No production code still depends on version-backed sprint behaviour.

* The dedicated sprint model has been renamedEnd of the insertion Begin of the deletion namespaceEnd of the deletion from Begin of the insertion`Agile::Sprint` to `Sprint`.

* Associations, queries, services, contracts, and APIs useEnd of the insertion the new Begin of the insertioncanonical sprint model consistently.

* Relevant sprint and Backlogs specs pass after the transition.End of the insertion Begin of the deletion Sprint model, effectively replacing it.End of the deletion