Remove public email check endpoint

[raymondjacobson]

Summary

• remove the unauthenticated identity-service /users/check route
• remove web/mobile/common/libs callers and email-in-use query plumbing
• keep guest account completion through the existing explicit guestEmail localStorage/query-param sign-up path instead of discovering guest status by arbitrary email lookup
• add a regression assertion that /users/check now 404s

Validation

• git diff --check
• node --check packages/identity-service/src/routes/user.js
• node --check packages/identity-service/test/expressAppTest.js
• npm run typecheck -w @audius/common -- --pretty false could not run because this worktree has no installed tsc (node_modules absent)
• attempted npm test -w identity-service -- --grep "does not expose email registration checks", but the wrapper does not support --grep and failed during Docker Postgres startup before tests ran; created test containers were removed

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Latest commit: ac5d837

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[github-actions]

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