Clean up Backlogs naming and namespacing for consistency, readability, and better alignment with Rails conventions.

This includes:

* removing remaining `Rb*` / `rb_*` prefixes from controllers, helpers, views, routes, and specs (some prefixes will already have been removed as part of <mention class="mention" data-id="72661" data-type="work_package" data-text="#72661">#72661</mention>

* namespacing Backlogs controllers and other applicable code consistently under Backlogs

* removing transitional prefixes and terminology such as `agile_sprint`, `new_*`, and `legacy_*` where they no longer reflect the actual architecture

* aligning filenames, constants, and related test names with the final structure


### Out of scope

Begin of the insertion* End of the insertion Do not Begin of the insertionchange namespacing of new models (we will continue to use `Agile::` rather than `Backlogs::` namepsace)

* Do notEnd of the insertion namespace legacy models - these will be removed in <mention class="mention" data-id="71254" data-type="work_package" Begin of the insertiondata-text="#71254">#71254</mention>

* There is a lot of overlap with #72661:End of the insertion Begin of the deletion data-text="#71254">#71254</mention>&nbsp;

End of the deletion Route-affecting action renames should Begin of the insertionhappen as part of that work package.End of the insertion Begin of the deletion be coordinated with <mention class="mention" data-id="72661" data-type="work_package" data-text="#72661">#72661</mention> , since those changes are mechanically coupled to route consolidation.End of the deletion