Add website analytics

A Tianji-style Websites section: a small tracking script, a public collection endpoint, and a
page per website with visitors, page views, visits, bounce rate, a chart and top lists for pages,
referrers, browsers, systems, devices, countries, languages and events. No cookies and no stored
IP address or user agent. SANDBAR_TRUST_PROXY makes the client address come from the proxy.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

.env.example

@@ -30,9 +30,14 @@ SANDBAR_MASTER_KEY=
 #SANDBAR_UPDATE_SERVER=https://git.veillax.com
 #SANDBAR_UPDATE_REPO=Veillax/sandbar
 
+# Behind a reverse proxy, take the client address from X-Forwarded-For. Needed for website analytics and
+# the sign-in limit to tell visitors apart. Only set it when the proxy is the only way to reach Sandbar.
+#SANDBAR_TRUST_PROXY=1
+
 # How long topic messages and monitor results are kept, in days.
 #SANDBAR_MESSAGE_DAYS=30
 #SANDBAR_CHECK_DAYS=30
+#SANDBAR_ANALYTICS_DAYS=365
 
 # Outgoing email, for email channels and for paging people by email.
 #SANDBAR_SMTP_HOST=smtp.example.com