Add Core RPC section to admin Stats tab

Surface invite diagnostics (RPC health, per-peer state, queues,
recommendations) under Resource Usage via /api/stats core payload
and auto-refreshing renderCoreStats in the stats UI.

includes/admin/admin-backend/routes/stats.js

@@ -304,6 +304,17 @@ async function handleStatsRoutes(req, res) {
       // Collect Hyperdrive stats
       const hyperdriveStats = collectHyperdriveStats();
       stats.hyperdrive = hyperdriveStats;
+
+      if (typeof state.diagnoseInviteIssues === 'function') {
+        try {
+          stats.core = state.diagnoseInviteIssues();
+        } catch (err) {
+          logDebug('Admin', `Core RPC stats unavailable: ${err.message}`);
+          stats.core = { initializing: true, error: err.message };
+        }
+      } else {
+        stats.core = { initializing: true };
+      }
       
       const responseTime = Date.now() - startTime;
       trackRequestWithTiming('/api/stats', true, responseTime);