cleanup: remove legacy backend and npm-app #5026

Job	Run time
build-and-check	2m 3s
test-backend	41s
test-cli	1m 1s
test-npm-app	38s
test-packages/agent-runtime	41s
test-packages/internal	37s
test-common	40s
test-.agents	47s
test-packages/billing	44s
test-integration-cli	42s
test-web	42s
test-integration-packages/agent-runtime	41s
test-integration-.agents	1m 11s
test-integration-npm-app	40s
test-integration-backend	42s
test-integration-common	38s
test-integration-web	38s
test-integration-packages/billing	42s
test-sdk	50s
test-integration-sdk	55s
test-integration-packages/internal	39s
	16m 52s

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