serve static files via whitenoise under gunicorn

Admin/DRF/Swagger CSS and JS never load under gunicorn since Django's
staticfiles app only auto-serves through `runserver`. WHITENOISE_USE_FINDERS
is tied to DEBUG so no collectstatic step is needed while DEBUG=1.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

requirements.txt

@@ -4,6 +4,7 @@ drf-spectacular==0.30.0
 requests==2.33.1
 redis==5.2.1
 python-dotenv==1.0.1
+whitenoise==6.11.0
 
 # RAG / AI tutor (اپ rag). کلاینت‌ها lazy import می‌شن؛ نبودِ این‌ها فقط
 # مسیرِ RAG رو از کار می‌ندازه، نه بقیه‌ی پروژه رو.