chore(deps): update dependency flask to v3.1.1 (#130)

This PR contains the following updates:

| Package | Change | Age | Confidence |
|---|---|---|---|
| [Flask](https://github.com/pallets/flask) ([changelog](https://flask.palletsprojects.com/page/changes/)) | `==3.1.0` -> `==3.1.1` | [![age](https://developer.mend.io/api/mc/badges/age/pypi/flask/3.1.1?slim=true)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://developer.mend.io/api/mc/badges/confidence/pypi/flask/3.1.0/3.1.1?slim=true)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

<details>
<summary>pallets/flask (Flask)</summary>

### [`v3.1.1`](https://github.com/pallets/flask/blob/HEAD/CHANGES.rst#Version-311)

[Compare Source](https://github.com/pallets/flask/compare/3.1.0...3.1.1)

Released 2025-05-13

- Fix signing key selection order when key rotation is enabled via
  `SECRET_KEY_FALLBACKS`. :ghsa:`4grg-w6v8-c28g`
- Fix type hint for `cli_runner.invoke`. :issue:`5645`
- `flask --help` loads the app and plugins first to make sure all commands
  are shown. :issue:`5673`
- Mark sans-io base class as being able to handle views that return
  `AsyncIterable`. This is not accurate for Flask, but makes typing easier
  for Quart. :pr:`5659`

</details>

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Reviewed-on: luke/epage#130
Reviewed-by: Luke Tainton <luke@tainton.uk>
Co-authored-by: Renovate [BOT] <renovate-bot@git.tainton.uk>
Co-committed-by: Renovate [BOT] <renovate-bot@git.tainton.uk>

.ARCHIVE/requirements.txt

@@ -1,7 +1,7 @@
 certifi==2025.6.15
 charset-normalizer==3.4.2
 click==8.2.1
-Flask==3.1.0
+Flask==3.1.1
 Flask-WTF==1.2.2
 idna==3.10
 itsdangerous==2.2.0