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/src/auth/providers/base.provider.js

@@ -37,7 +37,7 @@ class BaseProvider {
   }
 
   // Complete an SSO redirect flow: exchange the callback code for a normalized
-  // user profile ({ subject, email, name }).
+  // user profile ({ subject, email, emailVerified, name }).
   // eslint-disable-next-line no-unused-vars
   async handleCallback(params) {
     throw new Error(`handleCallback() not implemented for provider '${this.id}'`)
@@ -49,7 +49,7 @@ class BaseProvider {
     throw new Error(`getUserProfile() not implemented for provider '${this.id}'`)
   }
 
-  // Normalize a raw external profile to { subject, email, name }.
+  // Normalize a raw external profile to { subject, email, emailVerified, name }.
   // eslint-disable-next-line no-unused-vars
   mapUser(profile) {
     throw new Error(`mapUser() not implemented for provider '${this.id}'`)