Update disavowed event lookup to load one record

[aduth]

🛠 Summary of changes

Updates EventDisavowal::FindDisavowedEvent#event to avoid loading all events matching token fingerprints.

This is the result of a discussion with @jmhooper stemming from surprise around performance characteristics noted in #2855. While #first would still be advisable to avoid here due to built-in ordering ("if no order is defined it will order by primary key"), #find_by will locate at most one record without imposing any order. #take would have also worked.

The previous implementation was likely fine enough since the logic producing fingerprints would likely only ever produce 0 or 1 database results, but these revisions are simpler and more semantic, and should hopefully avoid future confusion around unoptimized LIMIT 1 queries.

📜 Testing Plan

Existing tests provide coverage for (a) missing, (b) matched, (c) matched old key. Verify they pass:

rspec spec/services/event_disavowal/find_disavowed_event_spec.rb