httpd: raise the default web session timeout to one hour

200 seconds was too short in practice: the UI stops polling while its
tab is hidden, so looking at another tab for a few minutes ended the
session. Make it 3600 seconds; "session <seconds>" still overrides it.

CHANGELOG.md

@@ -23,6 +23,11 @@
 
 ## Changed
 
+- Web UI
+  - The web session expires after one hour without a request (was 200 seconds). The UI stops
+    polling while its tab is hidden, so with 200 seconds switching tabs for a few minutes logged
+    you out. `session <seconds>` still sets it.
+
 ## Fixed
 
 - Time