The cancel route writes status 'canceled' but the supervisor compared against
'cancelled', so its "a route stopped it -> kill the child, free the asset"
branch was dead code. A canceled job's child ran to completion and wrote 'done'
unconditionally over the route's 'canceled'; since 'done' holds the asset but
'canceled' does not, a later delete released the asset a second time, dropping
ref_count below the live holder count and deleting a file still in use.
Add app/downloads/status.py as the single source of truth for the lifecycle
literals (JobStatus StrEnum + the HOLDING / STOPPED_BY_ROUTE sets) and use it at
the safety-critical sites: the supervisor stop check, the route cancel, and
release_asset's holding set. Make the worker's terminal done/error writes
conditional on the row still being 'running' so a child finishing just after a
route cancel/pause can no longer resurrect it. Add the missing 'canceled'
supervisor regression test.
(The full literal sweep across the downloads surface is left for the E3
downloads owner-unification; this is the data-safety subset.)
peter
367bcebc9f