Installer prerequisites: REQ-D through REQ-G

REQ-D: restore waitress and tzdata to requirements.in. They existed ONLY in the
generated requirements.txt (hand-added in bf9e60e), so the next
`uv pip compile` would have silently removed the WSGI server and the IANA
timezone database from every Windows install.

REQ-E: split production and development requirements. requirements.txt was
installing pytest, pytest-cov, pytest-flask, coverage, iniconfig and pluggy onto
production servers. Verified on a real Windows Server box before this change.
CI, scripts/test-external-plugin.sh and the dev docs now use requirements-dev.txt.

REQ-F: standardise on Python 3.14. The repo declared four different versions
(Dockerfile 3.12, DEPLOY-WINDOWS-IIS 3.12, INSTALL-WINDOWS-IIS 3.13, CI 3.13,
plus README, web.config and PLUGIN-EXTERNAL-REPO). 3.14 is in active bugfix
support until ~Apr 2027 and supported to Oct 2030; 3.13 entered security-only in
Apr 2026. All four compiled dependencies publish win_amd64 wheels for 3.14
(cryptography via an abi3 wheel), verified by building an offline wheelhouse and
installing it on Windows Server 2025.

REQ-G: state MySQL 8.0 as the standard for new installs; 5.7+/5.6 remain
supported on an existing server.

Lockfiles regenerated with uv pip compile. Production deps 44 -> 38.

deploy/windows/web.config

@@ -11,7 +11,7 @@
     - HttpPlatformHandler IIS module installed
       (https://www.iis.net/downloads/microsoft/httpplatformhandler)
     - URL Rewrite module installed (only for the optional X-Forwarded-For rule)
-    - Python 3.12 + a venv at APP_ROOT\venv with requirements.txt + waitress
+    - Python 3.14 + a venv at APP_ROOT\venv with requirements.txt + waitress
     - Secrets live in APP_ROOT\.env (wsgi.py load_dotenv() reads it). Keep them
       OUT of this file. Lock .env ACLs to the app pool identity + admins.