CHW-4123 EmailAliasExists

Default message: An account already exists for the underlying email address. Sign in or use a different email.
HTTP status: 409 (client)

Problem. NO LONGER EMITTED. Sign-up used to answer this code when the submitted address was a +tag alias of an already-registered one. It now answers CHW-4102 for both the exact-match and the alias case, byte-identically. Cause. The split was a wider existence oracle than the exact-match branch — one probe covered a whole +tag family — and the choice of code itself disclosed that an account had registered with a sub-address. Fix. Handle CHW-4102 instead: sign in to the existing account, reset its password, or sign up with a different address. This code stays registered so that a rolling deploy where an older pod still emits it remains documented and this page does not disappear; a client that still branches on it should treat it exactly as CHW-4102. HTTP 409.