Removes the `S3ContentKey` property from the `DbMission` entity and related DTOs and services.
This change streamlines the mission creation process and simplifies the data model.
The content is no longer stored separately.
Improves repository structure by introducing a generic DbCrud class and dedicated repositories for each entity.
This change enhances code maintainability and testability by separating concerns.
It also introduces soft delete functionality to the generic repository.
