feat(auth): unique, changeable, verifiable email addresses (engagement Phase 1b)

Makes `users.email` unique, de-duplicates the addresses an upgrade will find,
and builds the self-service change-and-verify flow that did not exist.

The uniqueness index is on a generated `email_norm AS (LOWER(email)) STORED`
column under `utf8mb4_bin`, NOT on `email` under a `_ci` collation as the plan
specified. Every case-insensitive collation this server offers is also
accent-insensitive: `josé@x.com` and `jose@x.com` compare equal, and those are
two different mailboxes. The plan's index would have refused the second address
forever and the de-duplication would have nulled a legitimate account's.

A requested address is STAGED in `email_pending` and only a tokened link
installs it, so a typo cannot silently redirect account-recovery mail.

`isDuplicateUsername()` now distinguishes the two indexes. All five call sites
branch on it; each answers differently on purpose, because a public form, an
IdP callback, a half-completed invite and an admin screen do not owe the same
person the same amount of truth.

SSO reads the IdP's actual `email_verified`/`verified` claim instead of
inferring verification from an address merely being present.

Co-Authored-By: Claude <noreply@anthropic.com>

server/test/authMe.test.js

@@ -24,6 +24,12 @@ test('/auth/me/account* rejects unauthenticated callers with 401', async () => {
       ['GET', '/api/v1/auth/me/account/identities'],
       ['PATCH', '/api/v1/auth/me/account/username', { username: 'someone' }],
       ['PATCH', '/api/v1/auth/me/account/password', { newPassword: 'abcd1234' }],
+      // Engagement Phase 1b — the email change/verify request half is self-service
+      // and must be gated exactly like the rest. (The CONFIRM half is public by
+      // design and lives at /auth/email/verify/:token, tested separately.)
+      ['PATCH', '/api/v1/auth/me/account/email', { email: 'new@example.com' }],
+      ['POST', '/api/v1/auth/me/account/email/resend'],
+      ['DELETE', '/api/v1/auth/me/account/email/pending'],
       ['POST', '/api/v1/auth/me/account/totp/setup'],
       ['POST', '/api/v1/auth/me/account/totp/enable', { code: '123456' }],
       ['DELETE', '/api/v1/auth/me/account/identities/google'],