Call handing a task to another user reassignment, not delegation, so the word is free for delegating authority

Orchestra's inbox lets a manager hand any task to another user, and lets a task's current assignee do the same with their own. The docs and the code call the second one "delegation": docs/human-tasks.md has a "Reassignment and delegation" section, docs/roadmap.md a "Task reassignment and delegation" bullet, and ReassignForm, InboxController and WorkItemManager::reassign() repeat it in their docblocks.

It is the same operation either way: ownership moves to the new assignee, immediately and permanently. Nothing is time-bounded, nothing comes back, and nothing records that the new holder acted for the previous one. That is reassignment.

Calling it delegation spends the word on a concept Orchestra does not have yet, and two real ones are coming: standing absence cover, where a deputy acts for someone for a period, and handing out a task while keeping the decision. Both are properly called delegation, and neither can be named while the word already means reassignment.

This issue changes wording only, with no behavior change:

• the "Reassignment and delegation" section in docs/human-tasks.md, and the sentence saying an assignee may delegate their own the same way
• the roadmap bullet
• the docblocks and inline comments in ReassignForm, InboxController::reassignAccess() and WorkItemManager::reassign()
• the French translations of the strings that changed