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.

docs/PLUGIN-EXTERNAL-REPO.md

@@ -85,7 +85,7 @@ ln -s ../../wjsf-shipping plugins/shipping
 
 # 3. Set up the framework as usual.
 python3 -m venv venv
-venv/bin/pip install -r requirements.txt
+venv/bin/pip install -r requirements-dev.txt
 
 # 4. Install (enable) your plugin.
 venv/bin/flask plugin install shipping
@@ -239,7 +239,7 @@ else
   python3 -m venv "$WORKDIR/venv"
   PYTHON="$WORKDIR/venv/bin/python"
   "$PYTHON" -m pip install --upgrade pip >/dev/null
-  "$PYTHON" -m pip install -r "$FRAMEWORK/requirements.txt"
+  "$PYTHON" -m pip install -r "$FRAMEWORK/requirements-dev.txt"
 fi
 
 echo "==> Linking plugin '$PLUGIN_NAME' into framework plugins/"
@@ -330,7 +330,7 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@v5
         with:
-          python-version: '3.12'
+          python-version: '3.14'
       - name: Fetch the harness from the framework
         run: |
           git clone --depth 1 --branch "$FRAMEWORK_REF" "$FRAMEWORK_URL" /tmp/framework