Add POST /jobs/{id}/retry to resubmit a failed or cancelled job

There is no POST /jobs/{id}/retry. A client that wants to resubmit a failed or cancelled job today has to have kept its own copy of the original duration_seconds/steps/callback_url and issue a brand new POST /jobs from scratch -- and even if a route existed, it couldn't reconstruct those parameters on its own, because the job document doesn't persist the parameters it was submitted with at all. Once a job is running, its own original submission is already gone from anywhere the server can see it.

app/store.py's create() never writes duration_seconds/steps/fail into the document it stores, and app/schemas.py's JobDocument doesn't declare them either.

1. pytest practice_tickets/tests/test_ticket_12_retry_endpoint_missing.py -v
2. Observe: POST /jobs/{id}/retry returns 404 -- the route doesn't exist yet, and the job document doesn't even persist the original submission's parameters to retry with.

This ticket is really two things stacked on top of each other: the missing route, and the missing data it would need. Implementing the route without first fixing the data gap isn't possible -- there's nowhere to read the original duration_seconds/steps back from. That makes this a good exercise in "the visible ask is smaller than the actual gap" -- the honest scope here starts one layer below where the route itself lives.

Start with what app/store.py's create() writes into a job document today, and what POST /jobs's currently has available (, , , the callback URL) that never makes it into that document. Once those are actually persisted, becomes: read the original document, reject if it isn't terminal, otherwise call the same job-creation path already uses with the original's parameters.