Allow HTTP_X_FORWARDED_FOR at server level instead of declaring it per-application

The stage 5 smoke test failure was a locked config section, but not one of the
two the installer unlocks. A diagnostic collected from the server returned:

  HTTP 500.52 - URL Rewrite Module Error
  Module RewriteModule, Handler httpplatformhandler
  Error Code 0x80070021
  Config Error: This configuration section cannot be used at this path.
  Config File: \\?\C:\shopdb-flask\web.config

handlers and httpPlatform were both overrideMode Allow and locked false, so
the unlock had worked. The section at fault was a third one,
system.webServer/rewrite/allowedServerVariables, which ships
overrideModeDefault="Deny". web.config declared <allowedServerVariables>
locally for the X-Forwarded-For rule, and IIS rejects that declaration
outright, failing the entire configuration before httpPlatformHandler ran.
python was therefore never launched and C:\shopdb-flask\logs stayed empty,
which reads as a dead application or a permissions fault and is neither.

Unlocking the section would let every site on the machine declare arbitrary
server variables. The installer now adds the single variable to the
server-level allow list, checking first because a duplicate add is an error,
and web.config no longer declares it. The rewrite rule is unchanged.

Verified by applying the installer's own uncommenting to the template and
parsing the result: one rewrite element, no allowedServerVariables, the rule
still setting HTTP_X_FORWARDED_FOR from REMOTE_ADDR.

shopdb-diagnose.py checked only the two sections the installer unlocks, so it
could not have named this one; the IIS error page did. It now reports the
lock state of the rewrite sections as well.

deploy/windows/web.config

@@ -67,15 +67,28 @@
       It ships DISABLED because it needs the URL Rewrite module; enabled without
       it, IIS returns HTTP 500.19 ("configuration section not well-formed").
 
+      There is deliberately NO <allowedServerVariables> block below. Setting a
+      server variable requires that variable to be allowed, but the section
+      system.webServer/rewrite/allowedServerVariables ships with
+      overrideModeDefault="Deny", so declaring it in an application's own
+      web.config is refused outright: IIS answered 500.52 with error 0x80070021,
+      "this configuration section cannot be used at this path", BEFORE it ever
+      reached httpPlatformHandler - so python was never launched and the stdout
+      log stayed empty, which looks like an application fault and is not one.
+
+      The installer instead allows the single variable at server level, which
+      grants exactly HTTP_X_FORWARDED_FOR rather than unlocking the section and
+      letting every site on the machine declare arbitrary server variables.
+
       The installer handles both: -ClientIpSource direct installs URL Rewrite
-      from the bundle and enables this; -ClientIpSource proxy leaves it alone.
-      By hand: install URL Rewrite, then delete the two marker lines below.
+      from the bundle, allows the variable, and enables this; -ClientIpSource
+      proxy leaves it alone. By hand: install URL Rewrite, run
+        appcmd set config /section:system.webServer/rewrite/allowedServerVariables ^
+          /+"[name='HTTP_X_FORWARDED_FOR']" /commit:apphost
+      then delete the two marker lines below.
     -->
     <!-- SHOPDB-CLIENTIP-BEGIN
     <rewrite>
-      <allowedServerVariables>
-        <add name="HTTP_X_FORWARDED_FOR" />
-      </allowedServerVariables>
       <rules>
         <rule name="Set X-Forwarded-For" stopProcessing="false">
           <match url=".*" />