feat: admin grant/revoke from admin panel

Add PUT /v1/admin/users/{id}/admin endpoint and frontend UI for
granting and revoking platform admin status. Uses atomic conditional
SQL (RevokeUserAdmin) to prevent race conditions that could remove
the last admin. Includes idempotency check, audit logging, and
confirmation dialog with self-demotion warning.

pkg/db/users.sql.go

@@ -415,6 +415,21 @@ func (q *Queries) ListUsersAdmin(ctx context.Context, arg ListUsersAdminParams)
 	return items, nil
 }
 
+const revokeUserAdmin = `-- name: RevokeUserAdmin :execrows
+UPDATE users u SET is_admin = false, updated_at = NOW()
+WHERE u.id = $1
+  AND u.is_admin = true
+  AND (SELECT COUNT(*) FROM users WHERE is_admin = true AND status != 'deleted') > 1
+`
+
+func (q *Queries) RevokeUserAdmin(ctx context.Context, id pgtype.UUID) (int64, error) {
+	result, err := q.db.Exec(ctx, revokeUserAdmin, id)
+	if err != nil {
+		return 0, err
+	}
+	return result.RowsAffected(), nil
+}
+
 const searchUsersByEmailPrefix = `-- name: SearchUsersByEmailPrefix :many
 SELECT id, email FROM users WHERE email LIKE $1 || '%' ORDER BY email LIMIT 10
 `