feat: add admin-only status page with cache stats

- New /api/dashboard/status endpoint (admin-only, 403 for non-admins)
- Returns server info (uptime, Node version, memory usage)
- Returns polling mode and interval
- Returns cache stats: entry count, total size, per-key breakdown
  with item count, size in KB, and TTL remaining
- Status button in admin controls header
- Collapsible status panel with grid layout
- Responsive: single column on mobile

server/utils/cache.js

@@ -29,6 +29,40 @@ class MemoryCache {
   clear() {
     this.store.clear();
   }
+
+  getStats() {
+    const now = Date.now();
+    const entries = [];
+    let totalSize = 0;
+
+    for (const [key, entry] of this.store.entries()) {
+      const json = JSON.stringify(entry.value);
+      const sizeBytes = Buffer.byteLength(json, 'utf8');
+      totalSize += sizeBytes;
+      const ttlRemaining = Math.max(0, entry.expiresAt - now);
+      const expired = now > entry.expiresAt;
+      let itemCount = null;
+      if (Array.isArray(entry.value)) {
+        itemCount = entry.value.length;
+      } else if (entry.value && typeof entry.value === 'object') {
+        if (Array.isArray(entry.value.records)) itemCount = entry.value.records.length;
+        else if (Array.isArray(entry.value.slots)) itemCount = entry.value.slots.length;
+      }
+      entries.push({
+        key,
+        sizeBytes,
+        itemCount,
+        ttlRemainingMs: ttlRemaining,
+        expired
+      });
+    }
+
+    return {
+      entryCount: this.store.size,
+      totalSizeBytes: totalSize,
+      entries
+    };
+  }
 }
 
 const cache = new MemoryCache();