cleanup: remove legacy backend and npm-app #5039

Job	Run time
build-and-check	2m 3s
test-integration-packages/agent-runtime	36s
test-integration-backend	47s
test-integration-packages/internal	46s
test-integration-.agents	1m 16s
test-.agents	39s
test-integration-cli	41s
test-integration-packages/billing	37s
test-integration-sdk	58s
test-integration-common	41s
test-sdk	48s
test-npm-app	45s
test-integration-npm-app	44s
test-packages/internal	45s
test-cli	1m 5s
test-packages/billing	38s
test-integration-web	42s
test-common	43s
test-backend	39s
test-packages/agent-runtime	55s
test-web	46s
	17m 34s

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