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/discord.provider.js

@@ -23,7 +23,14 @@ class DiscordProvider extends OAuth2Provider {
   }
   normalizeProfile(p = {}) {
     // global_name is the new display name; fall back to the legacy username.
-    return { subject: p.id, email: p.email || null, name: p.global_name || p.username || null }
+    return {
+      subject: p.id,
+      email: p.email || null,
+      // Discord spells the claim `verified` rather than `email_verified`, and it
+      // means exactly this: the user confirmed the address with Discord.
+      emailVerified: p.verified === true,
+      name: p.global_name || p.username || null,
+    }
   }
 }