pause/resume/cancel/delete/redownload each hand-rolled their asset bookkeeping
inline in the route (release, failed-asset reset, the 20-line fresh-asset
rebuild), duplicating logic the worker/GC also touch. Move each transition into a
service.* method (service.pause/resume/cancel/delete/redownload) that owns the
mutation + its commit; the routes keep only auth, the HTTP 409 guards, and
serialization. Drops the now-dead _set_status route helper and two stale imports.
Pure behaviour-preserving code motion (routes -56 lines). Green under the E3 S2
transition net + the full downloads suite (363 pass), ruff clean.