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/api/client.js

@@ -116,6 +116,18 @@ export const api = {
     req('/auth/me/account/username', { method: 'PATCH', body: { username } }),
   changePassword: (newPassword, currentPassword) =>
     req('/auth/me/account/password', { method: 'PATCH', body: { newPassword, currentPassword } }),
+  // Email address (engagement Phase 1b). changeEmail STAGES the address — the
+  // account keeps its current one until the emailed link is opened — so the UI
+  // must show `email_pending` as pending, never as the address in force.
+  changeEmail: (email, currentPassword) =>
+    req('/auth/me/account/email', { method: 'PATCH', body: { email, currentPassword } }),
+  resendEmailVerification: () => req('/auth/me/account/email/resend', { method: 'POST' }),
+  cancelEmailChange: () => req('/auth/me/account/email/pending', { method: 'DELETE' }),
+  // The confirm half is public and token-gated — it is reached from a mailbox,
+  // often with no session, so it deliberately sits outside /auth/me.
+  lookupEmailVerification: (token) => req(`/auth/email/verify/${encodeURIComponent(token)}`),
+  confirmEmailVerification: (token) =>
+    req(`/auth/email/verify/${encodeURIComponent(token)}`, { method: 'POST' }),
   totpSetup: () => req('/auth/me/account/totp/setup', { method: 'POST' }),
   totpEnable: (code) => req('/auth/me/account/totp/enable', { method: 'POST', body: { code } }),
   totpDisable: (code) => req('/auth/me/account/totp/disable', { method: 'POST', body: { code } }),
@@ -312,6 +324,12 @@ export const api = {
     createUser: (data) => req('/admin/users', { method: 'POST', body: data }),
     updateUser: (id, data) => req(`/admin/users/${id}`, { method: 'PUT', body: data }),
     deleteUser: (id) => req(`/admin/users/${id}`, { method: 'DELETE' }),
+    // Accounts whose address was cleared when addresses became unique (Phase 1b).
+    // They can still sign in but can receive no mail until they set a new one, so
+    // they are the list an operator has to work through.
+    emailDedupeReport: () => req('/admin/users/email-dedupe-report'),
+    acknowledgeEmailDedupeReport: () =>
+      req('/admin/users/email-dedupe-report/acknowledge', { method: 'POST' }),
     // A user's trusted devices + MFA reset (admin only).
     userTrustedDevices: (id) => req(`/admin/users/${id}/trusted-devices`),
     revokeUserTrustedDevice: (id, deviceId) =>