Pin every line in `requirements.txt`

[DevanshSrajput]

Context
Only pandas is pinned (to 2.2.3). The other ten deps float to
whatever is latest on PyPI at install time. That makes a clean
install today produce a different lockfile than a clean install
six months from now — and a different lockfile than the one
Streamlit Cloud is using right now. Reproducible builds need every
line pinned.

What to change
In requirements.txt, add ==X.Y.Z to every unpinned line. A
tested set is:

numpy==1.26.4
pandas==2.2.3          # already pinned
matplotlib==3.8.2
seaborn==0.13.2
PyPDF2==3.0.1
python-docx==1.1.0
Pillow==10.2.0
pytesseract==0.3.10
requests==2.31.0
httpx==0.27.0
streamlit==1.32.0
python-dotenv==1.0.1

Pick the lowest set that the test suite (python -m unittest discover -s tests -v) still passes under, and document the pin
date in a comment at the top of the file.

How to verify

• grep -E '^[a-zA-Z]' requirements.txt | grep -v '==' | wc -l
  prints 0 (every line is pinned).
• pip install --upgrade-strategy=only-if-needed -r requirements.txt
  succeeds on a clean venv.
• python -m unittest discover -s tests -v is green.
• A fresh pip freeze matches requirements.txt line for line
  (modulo the pip/setuptools/wheel lines pip adds itself).

Skill: dependency management.
Estimated effort: M (testing the pins takes most of the time).