Add user list to admin panel and validated/guest badges on posts

Admin panel: new "Käyttäjät" section showing all registered users with
post count, likes count, email and join date.

Posts: submissions by logged-in users show a green "Vahvistettu" badge,
while guest submissions show a random code (e.g. #L01U51) for tracking.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

api.php

@@ -843,6 +843,28 @@ switch ($action) {
     case 'admin_check':
         ok(['loggedIn' => isAdmin()]);
 
+    case 'admin_users':
+        if (!isAdmin()) err('Unauthorized', 403);
+        $users = readData('users.json', []);
+        $posts = getOrInitPosts();
+        $result = [];
+        foreach ($users as $u) {
+            $nick = mb_strtolower($u['nickname']);
+            $postCount = 0;
+            foreach ($posts as $p) {
+                if (mb_strtolower($p['author'] ?? '') === $nick) $postCount++;
+            }
+            $result[] = [
+                'id'        => $u['id'],
+                'nickname'  => $u['nickname'],
+                'email'     => $u['email'] ?? '',
+                'created'   => $u['created'] ?? '',
+                'likes'     => count($u['likes'] ?? []),
+                'postCount' => $postCount,
+            ];
+        }
+        ok(['users' => $result]);
+
     // ─── Käyttäjätunnukset ─────────────────────────────────────
     case 'user_register':
         $nickname = trim($body['nickname'] ?? '');