Add POST /jobs/batch to submit more than one job at once

Submitting N jobs today means N separate POST /jobs round-trips from the client. For a caller that legitimately wants to kick off a batch -- e.g. transcoding every file in a folder someone just uploaded -- that's N times the network overhead and N times the places a partial failure can happen (job 3 of 10 fails to submit; now the caller has to reconcile which of the other 9 actually went through).

POST /jobs/batch matches the path pattern of the existing GET /jobs/{job_id} route (treating batch as a job id), but no route is registered for that path on POST.

1. pytest practice_tickets/tests/test_ticket_10_batch_submit_endpoint.py -v
2. Observe: POST /jobs/batch with 3 job specs returns 405 -- this route doesn't exist yet.

This is almost entirely a thin wrapper around the existing single-job path -- create_job()'s logic (validate, create the Redis document, register a handle, spawn the worker task) doesn't need to change at all, it just needs to run once per spec in the batch instead of once per request. The interesting design question is failure handling: does one invalid spec in a batch of 10 reject the whole batch, or only that one entry? (Acceptance criteria here only require rejecting a fully empty batch.)

app/schemas.py needs a request/response pair for the batch shape (a list of JobCreateRequest in, a list of job_ids out). app/routers/jobs.py's existing create_job() body is the reference for what "submit one job" actually does -- a batch route calling that same sequence of steps once per spec keeps the two paths from drifting apart.