cleanup: remove legacy backend and npm-app #5024

Job	Run time
build-and-check	1m 57s
test-npm-app	45s
test-common	38s
test-backend	38s
test-packages/agent-runtime	43s
test-.agents	41s
test-web	45s
test-packages/billing	38s
test-cli	1m 2s
test-packages/internal	49s
test-sdk	44s
test-integration-.agents	1m 14s
test-integration-cli	37s
test-integration-common	38s
test-integration-npm-app	41s
test-integration-sdk	58s
test-integration-packages/billing	41s
test-integration-backend	46s
test-integration-packages/agent-runtime	40s
test-integration-packages/internal	41s
test-integration-web	37s
	16m 53s

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