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/installer/shopdb-install.ps1

@@ -1915,6 +1915,39 @@ If not, re-run matching how the application is published today: pass
         } finally { $ErrorActionPreference = $prevEap }
     }
 
+    # URL Rewrite: a rule may only set a server variable that is ALLOWED, and
+    # system.webServer/rewrite/allowedServerVariables ships with
+    # overrideModeDefault="Deny". An <allowedServerVariables> block in the app's
+    # own web.config is therefore refused outright - IIS answered 500.52 with
+    # 0x80070021, "this configuration section cannot be used at this path",
+    # before it ever reached httpPlatformHandler. python was never launched, the
+    # stdout log stayed empty, and the stage 5 smoke test reported it as a dead
+    # application when the config had been rejected wholesale.
+    #
+    # Allow the ONE variable at server level rather than unlocking the section.
+    # Unlocking would let every site on this machine declare arbitrary server
+    # variables; this grants exactly HTTP_X_FORWARDED_FOR.
+    if (($ClientIpSource -eq 'direct') -and (-not $WhatIfOnly)) {
+        $varName = 'HTTP_X_FORWARDED_FOR'
+        $section = 'system.webServer/rewrite/allowedServerVariables'
+        $prevEap = $ErrorActionPreference
+        $ErrorActionPreference = 'Continue'
+        try {
+            # Adding a duplicate entry is an error, so look before writing.
+            $current = (& $appcmd list config /section:$section 2>&1 | Out-String)
+            if ($current -match [regex]::Escape($varName)) {
+                Write-Log "$varName is already allowed for URL Rewrite" 'OK'
+            } else {
+                Write-Log "allowing $varName for URL Rewrite (server level)"
+                & $appcmd set config /section:$section "/+[name='$varName']" /commit:apphost 2>&1 |
+                    ForEach-Object { Write-Log "    $_" }
+                if ($LASTEXITCODE -ne 0) {
+                    Write-Log "  could not allow $varName - IIS will return 500.52 and the app will not start" 'WARN'
+                }
+            }
+        } finally { $ErrorActionPreference = $prevEap }
+    }
+
     # Stage 2 stops the pool on an upgrade so its files can be replaced. Nothing
     # else starts it again, so the smoke test would fail against a stopped site
     # and report it as a broken install.