fix for phone generation on email signup

deploy/migrations/0088_users_signup_email.down.sql

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+DROP INDEX IF EXISTS users_signup_email_lower_unique_idx;
+ALTER TABLE public.users DROP COLUMN signup_email;

deploy/migrations/0088_users_signup_email.up.sql

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+-- Email-signup accounts (see internal/domain.EncodeEmailPhone) now get a
+-- short, normal-looking random "888" phone number in users.phone instead of
+-- storing the email-derived string there directly (that string only ever
+-- travels on the wire during auth.sendCode/signIn/signUp, exactly like a real
+-- phone number would). signup_email is the durable email->user reverse
+-- lookup that replaces the old "phone IS the email encoding" identity: it is
+-- what SendCode/SignIn use to find a returning email-signup account by the
+-- (deterministic) wire value decoded back to an email.
+ALTER TABLE public.users ADD COLUMN signup_email character varying(200) NOT NULL DEFAULT '';
+
+-- Case-insensitive, ignores the empty default so real phone-number accounts
+-- (the overwhelming majority) never touch this index.
+CREATE UNIQUE INDEX users_signup_email_lower_unique_idx ON public.users (lower(signup_email)) WHERE signup_email <> '';