api: persist model-pull jobs to disk (reuse updater disk-mirror) so restart doesn't 404 the poll

[thinmintdev]

What to build

Updater jobs are disk-mirrored to /var/lib/hal0/update-jobs and survive restart; model-pull jobs live only in app.state.model_pull_jobs, so a restart mid-pull 404s the poll — despite the docstring claiming "Mirror of the updater route shape." Extend the same disk-mirror pattern (same make_job primitive) to model-pull jobs.

Pointers: updater.py:114; routes/models.py:1230. Audit Q4.2.

Acceptance criteria

• Model-pull jobs mirrored to disk like updater jobs
• Polling a model-pull job after a restart returns status, not 404

Blocked by

None - can start immediately