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/routes.manifest.json

@@ -401,6 +401,14 @@
       "method": "DELETE",
       "path": "/api/v1/admin/users/:id/trusted-devices/:deviceId"
     },
+    {
+      "method": "GET",
+      "path": "/api/v1/admin/users/email-dedupe-report"
+    },
+    {
+      "method": "POST",
+      "path": "/api/v1/admin/users/email-dedupe-report/acknowledge"
+    },
     {
       "method": "GET",
       "path": "/api/v1/admin/wiki"
@@ -457,6 +465,14 @@
       "method": "GET",
       "path": "/api/v1/admin/wiki/tags"
     },
+    {
+      "method": "GET",
+      "path": "/api/v1/auth/email/verify/:token"
+    },
+    {
+      "method": "POST",
+      "path": "/api/v1/auth/email/verify/:token"
+    },
     {
       "method": "GET",
       "path": "/api/v1/auth/invite/:token"
@@ -485,6 +501,18 @@
       "method": "GET",
       "path": "/api/v1/auth/me/account"
     },
+    {
+      "method": "PATCH",
+      "path": "/api/v1/auth/me/account/email"
+    },
+    {
+      "method": "DELETE",
+      "path": "/api/v1/auth/me/account/email/pending"
+    },
+    {
+      "method": "POST",
+      "path": "/api/v1/auth/me/account/email/resend"
+    },
     {
       "method": "GET",
       "path": "/api/v1/auth/me/account/identities"