fix: avoid disclosing existing emails

[invinsibleDev]

Summary

• Return neutral success-shaped responses when signup or add-email receives an address already owned by an existing user.
• Preserve the existing flow for unclaimed EmailAddress nodes.
• Skip signup/email-verification mail sending when the resolver intentionally returns no nonce.

Fixes #1832

Validation

• git diff --check
• node --check backend/src/schema/resolvers/helpers/existingEmailAddress.js
• node --check backend/src/schema/resolvers/emails.js
• node --check backend/src/schema/resolvers/registration.js
• node --check backend/src/middleware/email/emailMiddleware.js

Full backend Jest was not run because corepack yarn install --frozen-lockfile is currently blocked by a 404 while fetching the old Marak/faker.js GitHub tarball.