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/player/PlayerAccount.jsx

@@ -4,6 +4,7 @@ import { Loading, ErrorState } from '../../components/PageState.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 { useAuth } from '../../contexts/AuthContext.jsx'
 import { api } from '../../api/client.js'
 
@@ -423,6 +424,7 @@ export default function PlayerAccount() {
             {account.email ? ` · ${account.email}` : ''}
           </p>
           <ChangeUsername account={account} onChanged={onUsernameChanged} />
+          <EmailAddressPanel account={account} reload={load} />
           <ChangePassword account={account} />
           <TwoFactor account={account} reload={load} />
           {account.totp_enabled && (