register rag app, RAG provider settings, autoload .env

settings.py
  - load .env via python-dotenv (override=False) so local runs and the RAG
    keys work without `export`; production/docker env still wins.
  - add 'rag' to INSTALLED_APPS.
  - new RAG / AI provider block: OPENAI_API_KEY / OPENAI_BASE_URL (GapGPT)
    / OPENAI_CHAT_MODEL / OPENAI_VISION_MODEL / OPENAI_EMBEDDING_MODEL /
    OPENAI_TIMEOUT_SECONDS, QDRANT_URL / QDRANT_API_KEY,
    RAG_COLLECTION_NAME_TEMPLATE / RAG_GRADE_TITLE_OVERRIDES /
    RAG_RETRIEVAL_TOP_K / RAG_FORCE_MOCK / RAG_ANSWER_CACHE_TTL.
  - add 'rag_ingest' to DEFAULT_THROTTLE_RATES.
  - under `manage.py test`: force RAG_FORCE_MOCK on and RAG_ANSWER_CACHE_TTL
    to 0 so no test hits OpenAI/Qdrant or a warm cache.
urls.py
  - mount rag.urls under /api/.
requirements.txt
  - python-dotenv; openai, qdrant-client; pymupdf / pdf2image / pillow
    (optional, only for PDF ingestion).
.env.example
  - document the OPENAI_* / QDRANT_* / RAG_* keys, the discovered Qdrant
    collection names, and the parked OPENAI_API_KEY_OPENAI_DIRECT slot
    (no provider-switch logic by design).

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

config/urls.py

@@ -51,6 +51,7 @@ urlpatterns = [
     path('api/', include('content.urls')),
     path('api/', include('progress.urls')),
     path('api/', include('ai_tutor.urls')),
+    path('api/', include('rag.urls')),
 
     # اپ‌های فاز ۲ / بخش ۱ (subscription, payment, discount)
     path('api/', include('subscription.urls')),