cleanup: remove legacy backend and npm-app #5062

Job	Run time
build-and-check	2m 1s
test-.agents	40s
test-common	40s
test-cli	1m 7s
test-backend	38s
test-packages/billing	44s
test-npm-app	38s
test-sdk	50s
test-web	50s
test-integration-packages/billing	37s
test-integration-web	40s
test-integration-npm-app	37s
test-integration-common	40s
test-packages/agent-runtime	51s
test-integration-packages/internal	46s
test-integration-packages/agent-runtime	47s
test-integration-.agents	1m 13s
test-integration-backend	37s
test-packages/internal	40s
test-integration-cli	46s
test-integration-sdk	1m 0s
	17m 22s

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