chore(deps): update python docker tag to v3.14 (#556)

* chore(deps): update python docker tag to v3.14

* ci: read the Python version from backend/.python-version

The hardcoded python-version let CI keep testing 3.13 while the Docker
image moved to 3.14. Reading the version file keeps CI and the shipped
runtime on the same interpreter permanently.

---------

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>
Co-authored-by: Tássio Noronha <tassionoronha@gmail.com>

Author: renovate[bot]

.github/workflows/ci.yml

@@ -49,7 +49,9 @@ jobs:
         if: steps.backend-changes.outputs.run_backend == 'true'
         uses: actions/setup-python@v7
         with:
-          python-version: "3.13"
+          # Single source of truth with the Docker image: CI must test the
+          # same interpreter the backend ships with.
+          python-version-file: backend/.python-version
           cache: pip
 
       - name: Install dependencies

backend/.python-version

@@ -1 +1 @@
-3.13
+3.14

backend/Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3.13-slim
+FROM python:3.14-slim
 
 WORKDIR /app