Upgrade to Python 3.14

[jgravois]

Currently, as the Dockerfile declares, the app uses Python 3.12.

Upgrade to Python 3.14: https://www.python.org/downloads/release/python-3144/

• django 5.2 definitely supports python 3.14.
• requests etc. appear to be compatible too

that said, more research will be needed to verify downstream compatibility

prior art: #53
prior discussion: #73 (comment)