cleanup: remove legacy backend and npm-app #5037

Job	Run time
build-and-check	2m 10s
test-common	37s
test-.agents	41s
test-cli	58s
test-packages/billing	40s
test-backend	39s
test-web	46s
test-sdk	46s
test-npm-app	38s
test-packages/internal	39s
test-packages/agent-runtime	44s
test-integration-sdk	1m 1s
test-integration-backend	52s
test-integration-common	36s
test-integration-packages/billing	39s
test-integration-web	37s
test-integration-npm-app	44s
test-integration-packages/internal	38s
test-integration-.agents	1m 12s
test-integration-packages/agent-runtime	48s
test-integration-cli	39s
	17m 4s

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