feat: Make OCR and task concurrency configurable #175

Job	Run time
build-docreader	5m 59s
build-app (arm64, linux/arm64, ubuntu-24.04-arm)	5m 37s
build-app (amd64, linux/amd64, ubuntu-latest)	5m 41s
build-ui	7m 30s
merge	17s
	25m 4s

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