feat(auth): center-scoped single admin + GET /admin-email + cli

Triage channels in Guilds need to deliver every message to a single
"observer" admin user across the whole Center deployment (regardless of
on-duty / mention). This adds the data + auth surface for that.

## Schema

`users.isAdmin TINYINT DEFAULT 0` (synchronize:true auto-applies; cli
`user set-admin` enforces at-most-one in a transaction).

## CLI (subject `user`)

- `set-admin --email <e>` — clears every other admin row first, then
  marks the target. Returns `{admin: {email, userId}}`
- `clear-admin` — unsets all (returns `{cleared: N}`)
- `show-admin` — prints `{admin: {email, userId}}` or `{admin: null}`

## HTTP

`GET /auth/admin-email` (NOT @Public — requires a guild-node api key
via the existing CenterApiKeyGuard). Returns:
  - `{email: "...", userId: "..."}` if an admin exists
  - `null` (literal JSON) if no admin

Guild backends cache the result (1 day TTL per spec; with cli refresh
override on guild side).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/auth/auth.service.ts

@@ -204,6 +204,56 @@ export class AuthService {
     return { guilds, guildAccessTokens };
   }
 
+  // -------------------------------------------------------------------
+  // Center-scoped admin (at most one per deployment).
+  //
+  // setAdmin / clearAdmin are reachable only via cli, not HTTP — admin
+  // promotion bypasses every HTTP gate (the admin observes ALL triage
+  // traffic) so we don't want it tunneled through any token / api-key.
+  //
+  // getAdminEmail is exposed to guild backends (api-key auth) so each
+  // guild can mark the admin as a triage observer.
+  // -------------------------------------------------------------------
+
+  async getAdminEmail(): Promise<{ email: string; userId: string } | null> {
+    const admin = await this.userRepo.findOne({ where: { isAdmin: true } });
+    if (!admin) return null;
+    return { email: admin.email, userId: admin.id };
+  }
+
+  async setAdmin(email: string): Promise<{ email: string; userId: string }> {
+    const target = await this.userRepo.findOne({ where: { email } });
+    if (!target) {
+      throw new UnauthorizedException(`user ${email} not found`);
+    }
+    // Enforce at-most-one-admin: clear every other row first, then set
+    // the target. Two UPDATEs in one txn so a peer never sees two admins.
+    await this.userRepo.manager.transaction(async (txn) => {
+      await txn.update(User, {}, { isAdmin: false });
+      await txn.update(User, { id: target.id }, { isAdmin: true });
+    });
+    await this.audit.write({
+      action: 'admin.set',
+      actorId: target.id,
+      targetType: 'user',
+      targetId: target.id,
+      detail: JSON.stringify({ email: target.email }),
+    });
+    return { email: target.email, userId: target.id };
+  }
+
+  async clearAdmin(): Promise<{ cleared: number }> {
+    const res = await this.userRepo.update({ isAdmin: true }, { isAdmin: false });
+    await this.audit.write({
+      action: 'admin.clear',
+      actorId: 'cli',
+      targetType: 'user',
+      targetId: 'all',
+      detail: JSON.stringify({ cleared: res.affected ?? 0 }),
+    });
+    return { cleared: res.affected ?? 0 };
+  }
+
   async register(input: RegisterDto) {
     const exists = await this.userRepo.findOne({ where: { email: input.email } });
     if (exists) {