Remove the CSV build cap #902

Summary

The CSV build cap limits each API process to a couple of large score set CSV builds at once, because a build is CPU-bound and several at once stall the process. It's a stopgap. Once exports are cheap or served from cache it only turns downloads away, so this issue removes it.

Scope

• Delete lib/csv/build_limiter.py and its uses in the three CSV routes in routers/score_sets.py.
• Remove CSV_BUILD_SLOTS and CSV_BUILD_MIN_ROWS from the README and the API compose files in mavedb-deployment.
• Remove the 503 note from the Quickstart's Rate Limits section, if nothing else still returns it for downloads.

Before removing

• csv_build_refused hasn't appeared in the prod request logs for two weeks after Hoist per-row namespace resolution and batch the ClinVar fetch in CSV export #845 and Cache public CSV responses (first consumer of the API cache) #771 are deployed.

Acceptance criteria

• No score set CSV route returns 503 when other builds are running.
• A load test of 10 concurrent full downloads of the largest score set doesn't slow down other routes.