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/SelfHostedService.php

@@ -22,7 +22,22 @@ class SelfHostedService extends Model implements HasMedia
     use HasTags;
     use InteractsWithMedia;
 
-    protected $guarded = [];
+    /**
+     * @var array<int, string>
+     */
+    protected $fillable = [
+        'name',
+        'slug',
+        'type',
+        'intro',
+        'url_clearnet',
+        'url_onion',
+        'url_i2p',
+        'url_pkdns',
+        'ip',
+        'contact',
+        'anon',
+    ];
 
     protected $casts = [
         'id' => 'integer',