Add vector search API endpoint (#2815)

* Add vector search API endpoint

* Add missing import

* Modify the data that is returned

* Change similarityThreshold to scoreThreshold

As this is what is actually returned by the search

* Removing logging (oops!)

* chore: regen swagger docs for new endpoint
fix: update function to sanity check values to prevent crashes during search

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Co-authored-by: Scott Bowler <scott@dcsdigital.co.uk>