Pin Python dependencies in app/server/requirements.txt #234

Problem

requirements.txt has no version constraints:
ntextflask sqlalchemy flask_sqlalchemy n
This means builds can break silently when new major versions release (e.g. SQLAlchemy 1.x to 2.x had breaking API changes).

Fix

• Pin to compatible version ranges, e.g.:
  nflask>=3.0,<4.0 sqlalchemy>=2.0,<3.0 flask_sqlalchemy>=3.1,<4.0 n