cleanup: remove legacy backend and npm-app #5023

Job	Run time
build-and-check	2m 16s
test-integration-.agents	1m 15s
test-integration-web	39s
test-integration-packages/internal	41s
test-integration-cli	37s
test-integration-packages/billing	40s
test-integration-packages/agent-runtime	37s
test-integration-npm-app	39s
test-integration-sdk	51s
test-.agents	39s
test-backend	43s
test-npm-app	46s
test-integration-backend	39s
test-integration-common	39s
test-common	39s
test-packages/billing	39s
test-packages/internal	40s
test-web	44s
test-cli	56s
test-packages/agent-runtime	45s
test-sdk	47s
	16m 51s

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