bingo: add a reachability monitor for the status page

datacenterbingo gets a public page showing whether each datacenter's
network answers. The probing lives here, next to the schema and the
admin, since the public site only has a read-only role.

bin/monitor runs as a second container from the same image. Once a
minute it opens one TCP connection to every active target (a handshake
or a refusal both count as answered) and folds the results into
5-minute rows per datacenter, plus a per-bucket row for control hosts in
Russia, so a lost vantage point isn't mistaken for dark sites. Rows are
kept 30 days. Targets are managed at /admin/bingo/monitor; only hosts a
public source ties to that one site belong there.

Migration 017 also grants the new tables to the bingo role, so the
public site can read them without a storage redeploy.

Assisted-by: Claude:opus-5-5

views/admin/bingo/index.erb

@@ -2,7 +2,7 @@
 <div class="header">
     <h1>Bingo</h1>
     <p class="tagline">
-        <% if bingo_public_url %><a href="<%= bingo_public_url %>" target="_blank" rel="noopener">public card ↗</a><% end %>
+        <% if bingo_public_url %><a href="<%= bingo_public_url %>" target="_blank" rel="noopener">public card ↗</a> · <% end %><a href="/admin/bingo/monitor">monitor</a>
     </p>
 </div>