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>

client/src/routes/admin/views/AccountAdmin.jsx

@@ -4,6 +4,7 @@ import ProviderIcon from '../../../components/ProviderIcon.jsx'
 import RecoveryCodesDisplay from '../../../components/security/RecoveryCodesDisplay.jsx'
 import TrustedDevicesPanel from '../../../components/security/TrustedDevicesPanel.jsx'
 import RecoveryCodesPanel from '../../../components/security/RecoveryCodesPanel.jsx'
+import EmailAddressPanel from '../../../components/security/EmailAddressPanel.jsx'
 import { api } from '../../../api/client.js'
 
 // Link/unlink external SSO identities to this account. Linking redirects through
@@ -322,6 +323,10 @@ export default function AccountAdmin() {
         </>
       )}
 
+      {/* The self-service address, from the same component the player portal
+          renders — /auth/me/account is one surface for every role. */}
+      {account && <EmailAddressPanel account={account} reload={load} />}
+
       <LinkedAccounts />
     </section>
   )