ci: test on Python 3.13, the version every deployment actually runs

CI has been testing 3.11 while nothing ships it.  docker/api/Dockerfile and
docker/help/Dockerfile are python:3.13-trixie, docker/controller is
python:3.13-slim-trixie, Trixie's system interpreter is 3.13, and Debian 12 --
the last place 3.11 was current -- is pinned to the debian12-final tag and takes
no new code.  So every Python job here exercised an interpreter no user has.

All four CircleCI Python jobs and the GitHub Actions controller job move to 3.13.
The cache key goes with them: it stores the whole venv, so restoring a
deps-3.11 venv into a 3.13 container would hand it an interpreter that no longer
exists.

The disabled api-tests-3-12 job is deleted rather than repointed.  It was
switched off with the note "Sanic does not yet support 3.12", which 3.13 disproves,
and carrying a second version around is what let this drift open in the first
place.

Verified on 3.13.12 / pytest 9.0.3 before changing anything: the backend suite
passes (1709 passed, 5 skipped), alembic upgrade runs and `db check` reports no
new operations, and the controller suite passes bar two docker-mode dashboard
tests that fail identically on 3.11 here -- pre-existing and environment-dependent,
not a 3.13 regression.

`sanic-api-start` is the one job I could not verify locally: wrolpi/api_utils.py
sets `Sanic.start_method = "fork"` and macOS has defaulted to spawn since 3.8, so
that job fails here on any Python version.  Linux CI is unaffected -- fork is
still the default there on 3.13 -- and the test suite already runs the Sanic app
in-process, which is the substantive check.  Worth knowing that 3.14 changes the
Linux default away from fork, so that line has a deadline on it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.github/workflows/controller-tests.yml

@@ -21,6 +21,7 @@ jobs:
       - name: Setup Python
         uses: actions/setup-python@v5
         with:
+          # Trixie ships 3.13, and docker/controller/Dockerfile is python:3.13-slim-trixie.
           python-version: '3.13'
           cache: 'pip'
           cache-dependency-path: controller/requirements.txt