api: make num_threads configurable #4021

Job	Run time
api-check-needed	5s
e2e-check-needed	5s
Lint: ClientPrint (ESLint)	15s
frontend-check-needed	5s
Tests: PDF	17s
Lint: Print (ESLint)	23s
Tests: Print	59s
Lint: Frontend (ESLint)	29s
Tests: Frontend	3m 39s
Tests: API	4m 50s
Lint: API (php-cs-fixer)	34s
Lint: e2e (ESLint)	24s
Tests: End-to-end build / Tests: End-to-end (build job)	1m 1s
Tests: End-to-end run / Tests: End-to-end (chrome)	4m 11s
Tests: End-to-end run / Tests: End-to-end (firefox)	4m 35s
Tests: End-to-end run / Tests: End-to-end (edge)	4m 41s
Finish coveralls report	3s
workflow-success	4s
Send out CI success event	0s
	26m 40s

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