Port the v1 /tasks/{projecttask}/assignees routes to the Huma-backed
/api/v2. The resource self-registers (RegisterTaskAssigneeRoutes) and
reuses the model's Can* methods via the generic Do* handlers:
- POST /tasks/{projecttask}/assignees → assign a user (body: user_id)
- GET /tasks/{projecttask}/assignees → list assignees (as users)
- DELETE /tasks/{projecttask}/assignees/{user} → un-assign
The list element type is []*user.User (assignees are returned as the
assigned users), which differs from the create body (a TaskAssginee
carrying user_id); the list handler type-asserts to []*user.User.
create/delete require write access to the task's project, list requires
read — enforced at the model level.
The webtest re-proves the full v1 permission matrix on the v2 surface
(read-only shares forbidden, write/admin allowed for create and delete;
already-assigned, no-project-access, missing-user, and missing-task
error codes) so v1's routes can be removed later.
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