api: make DB engine pool size configurable and increase default

Jan Havlínrequested to merge
api-make-db-engine-pool-size-configurable into main

Part of TFT-3527.

This should get rid of sqlalchemy.exc.TimeoutError: QueuePool limit of size 5 overflow 10 reached, connection timed out, timeout 30.00 (Background on this error at: https://sqlalche.me/e/14/3o7r) errors due to too many API calls.

Edited by Jan Havlín

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