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.

docs/INSTALL-WINDOWS-IIS.md

@@ -13,7 +13,7 @@ lives). Run PowerShell as Administrator.
 
 | Need | Notes |
 | --- | --- |
-| **Python 3.12** (64-bit) | `python --version` |
+| **Python 3.13** (64-bit) | `python --version` |
 | **IIS** with **HttpPlatformHandler** | https://www.iis.net/downloads/microsoft/httpplatformhandler (direct MSI: `download.microsoft.com/download/8/1/3/813AC4E6-9203-4F7A-8DD5-F3D54D10C5CD/httpPlatformHandler_amd64.msi`) |
 | **MySQL 5.7+/8.0** (or 5.6 with the flags in step 1) | reachable from the app host |
 | URL Rewrite (optional) | only for the real-client-IP rule; skip it and the app still runs |