chore: prepare Heroku-26 stack upgrade

.python-version

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+3.12

README.md

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 # python-flask-server
+
+## Heroku deployment
+
+This app targets the `heroku-26` stack and Python 3.12. Heroku uses the
+`app.json` stack for Review Apps and Heroku CI, and `.python-version` keeps the
+Python major version consistent while allowing automatic patch updates.
+
+To upgrade the existing Cedar app, change its stack before the next deployment:
+
+```sh
+heroku stack:set heroku-26 --app easyeyes-python-flask-server
+```
+
+The next build uses Heroku-26. Confirm the deployed stack and Python version
+afterward:
+
+```sh
+heroku stack --app easyeyes-python-flask-server
+heroku run python --version --app easyeyes-python-flask-server
+```

app.json

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+{
+  "name": "EasyEyes Python Flask Server",
+  "stack": "heroku-26"
+}