cleanup: remove legacy backend and npm-app #5070

Job	Run time
build-and-check	2m 6s
test-integration-npm-app	42s
test-packages/agent-runtime	58s
test-cli	1m 19s
test-common	39s
test-integration-cli	47s
test-integration-.agents	1m 19s
test-web	50s
test-integration-web	39s
test-integration-backend	41s
test-integration-sdk	1m 2s
test-npm-app	52s
test-backend	40s
test-packages/billing	49s
test-integration-packages/internal	40s
test-.agents	40s
test-sdk	48s
test-integration-packages/agent-runtime	40s
test-packages/internal	41s
test-integration-common	40s
test-integration-packages/billing	39s
	18m 11s

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