refactor: define explicit fillable attributes on all Eloquent models

Replace $guarded usage with explicit $fillable arrays across all models.
Sync fillable definitions with current database schema and add tests.

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

app/Models/ScheduledTask.php

@@ -29,7 +29,14 @@ class ScheduledTask extends BaseModel
     use HasFactory;
     use HasSafeStringAttribute;
 
-    protected $guarded = [];
+    protected $fillable = [
+        'enabled',
+        'name',
+        'command',
+        'frequency',
+        'container',
+        'timeout',
+    ];
 
     public static function ownedByCurrentTeamAPI(int $teamId)
     {