Add a Circuit filter to the filter bar

Buttons are generated from CONFIG.circuits (not hand-written in
index.html), so adding a circuit to the config still only takes one
line — its filter button appears automatically. Uses accent color to
match the circuit badges on the cards, same active/dimmed styling as
Format/Scope. An event matches the circuit filter if it belongs to any
active circuit (OR within the group), not all of them, since an event
can legitimately carry more than one circuit tag.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SBzcNkW7JcYipnAX6HfgmK

sites/events/index.html

@@ -26,6 +26,8 @@
         <span class="text-xs uppercase tracking-widest opacity-50 ml-3 mr-1">Scope</span>
         <button type="button" class="btn btn-sm btn-info" data-filter-group="scope" data-filter-value="RTT">RTT</button>
         <button type="button" class="btn btn-sm btn-info" data-filter-group="scope" data-filter-value="GT">GT</button>
+        <span class="text-xs uppercase tracking-widest opacity-50 ml-3 mr-1">Circuit</span>
+        <span id="circuit-filters" class="contents"></span>
       </div>
 
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