Standardize on Python 3.13 (matches prod 3.13.7)

Prod runs Python 3.13.7, not the originally planned 3.12. Align the
stack: CI both jobs 3.12->3.13, the IIS install runbook and the dev
setup guide to 3.13 (winget Python.Python.3.13). NOTE for whoever
maintains the offline kit: its wheels are still cp312 and must be
regenerated as cp313 before the next air-gapped deploy.

.github/workflows/ci.yml

@@ -23,7 +23,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: actions/setup-python@v5
         with:
-          python-version: '3.12'
+          python-version: '3.13'
           cache: pip
       - run: pip install -r requirements.txt
       - run: python -m pytest -q
@@ -72,7 +72,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: actions/setup-python@v5
         with:
-          python-version: '3.12'
+          python-version: '3.13'
           cache: pip
       - run: pip install -r requirements.txt
       - name: Prime the CI database (utf8mb4 + native auth for pymysql)