limit concurrent downloads per user and queue the rest

[grojeda]

A user (identified by their recipient email) can now have at most 10 downloads in flight at once; anything beyond that is queued inside ogcapi and submitted to AWS Batch automatically as their slots free, rather than rejected — a queued download still gets a job ID and answers on the status endpoint. A slot stays occupied for the whole workflow (the master job plus the prepare/collect children DAS spawns), counted from one cached sweep of the Batch queues shared by all users. The execute and status responses gain queued and queuePosition so the portal can show the queued state without parsing message text; both additions are backwards compatible. Configurable under aws.batch.job.user-limit — max-concurrent defaults to 10, enabled: false turns it off. Note the recipient email is not authenticated, so this is a fairness mechanism rather than a security control.