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/Project.php

@@ -24,7 +24,10 @@ class Project extends BaseModel
     use HasFactory;
     use HasSafeStringAttribute;
 
-    protected $guarded = [];
+    protected $fillable = [
+        'name',
+        'description',
+    ];
 
     /**
      * Get query builder for projects owned by current team.