UFOs API: add `limit` param to `/records` for record samples

UFOs API keeps copies of the most recent 500 records, for every collection, seen in the firehose. These sample records are available over the API right now at /records, but that endpoint always limits the number of records returned to 42.

We should add a limit query parameter to that API endpoint, with

• minimum: 1
• default: 10
• maximum: 100

i'm open to increasing the max more! it's intentionally higher than what's exposed so that limited amounts of spam or other bad content can be purged without losing the whole cache. the internal limit can probably also be safely increased