cleanup: remove legacy backend and npm-app #5078

Job	Run time
build-and-check	2m 6s
test-integration-backend	35s
test-integration-.agents	1m 12s
test-integration-common	49s
test-integration-packages/internal	44s
test-integration-packages/agent-runtime	47s
test-integration-cli	39s
test-integration-npm-app	38s
test-integration-packages/billing	47s
test-integration-web	45s
test-integration-sdk	1m 13s
test-common	40s
test-packages/agent-runtime	51s
test-.agents	38s
test-packages/billing	40s
test-cli	1m 3s
test-packages/internal	37s
test-web	44s
test-sdk	52s
test-npm-app	40s
test-backend	40s
	17m 40s

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