security: high-severity fixes (api throttle, fillable, idor, path, rel)

- Add 60 req/min throttle to the public API group and a stricter 10 req/min
  throttle to POST /highscores.
- Replace mass-assigned $guarded=[] with explicit $fillable on User, Meetup,
  Course, Lecturer, and SelfHostedService. created_by stays out of the
  whitelist; the existing creating() hooks continue to populate it.
- Require authenticated user on Api/MeetupController::index instead of
  trusting the user_id query parameter (IDOR).
- Constrain the /img and /img-public route paths to a safe character set
  and reject any path containing ".." in ImageController.
- Add rel="noopener noreferrer" to every target="_blank" link on the meetup
  and course landing pages.

app/Models/Meetup.php

@@ -23,11 +23,25 @@ class Meetup extends Model implements HasMedia
     use InteractsWithMedia;
 
     /**
-     * The attributes that aren't mass assignable.
-     *
-     * @var array
+     * @var array<int, string>
      */
-    protected $guarded = [];
+    protected $fillable = [
+        'name',
+        'slug',
+        'city_id',
+        'intro',
+        'telegram_link',
+        'webpage',
+        'twitter_username',
+        'matrix_group',
+        'nostr',
+        'nostr_status',
+        'simplex',
+        'signal',
+        'community',
+        'github_data',
+        'visible_on_map',
+    ];
 
     /**
      * The attributes that should be cast to native types.