Fix build error: Add missing nltk dependency

🐛 Fixed:
- Added nltk==3.8.1 to requirements.txt
- Resolves ModuleNotFoundError when Dockerfile tries to download NLTK data
- Dockerfile line 16 requires nltk module for data download

The error occurred because:
- Dockerfile: RUN python -m nltk.downloader punkt stopwords
- requirements.txt: Missing nltk dependency
- requirements-full.txt: Has nltk but not used by Dockerfile

🤖 Generated with Claude Code

Co-Authored-By: Claude <[email protected]>

requirements.txt

@@ -2,4 +2,5 @@ fastapi==0.104.1
 uvicorn==0.24.0
 pydantic==2.5.0
 python-multipart==0.0.6
-aiofiles==23.2.1
+aiofiles==23.2.1
+nltk==3.8.1