cleanup: remove legacy backend and npm-app #5063

Job	Run time
build-and-check	2m 0s
test-integration-.agents	1m 3s
test-integration-packages/agent-runtime	47s
test-integration-backend	38s
test-integration-common	46s
test-integration-npm-app	39s
test-integration-cli	43s
test-integration-sdk	1m 1s
test-integration-packages/internal	44s
test-backend	39s
test-.agents	46s
test-integration-web	39s
test-packages/agent-runtime	44s
test-packages/billing	39s
test-packages/internal	39s
test-integration-packages/billing	42s
test-npm-app	44s
test-common	40s
test-cli	1m 18s
test-web	51s
test-sdk	46s
	17m 28s

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