Perf: parallelize admin loads + edge-cache /api/brief

Two concrete latency wins found by measuring (server compute is 2-17ms; the time
is in the path, not the box):
- Admin panel fired its 6 API calls SEQUENTIALLY (await chain) — so it paid the
  uncached origin round-trip six times back-to-back. Now one Promise.all batch.
  This is the admin lag.
- /api/brief (the home "Gathering the good news…" content) wasn't edge-cached, so
  a distant anonymous visitor triggered a Cloudflare→residential-origin pull.
  Same global/shareable boundary as /api/feed: public s-maxage=45 when no
  prefs/exclude, else private,no-store. (Needs /api/brief added to the CF cache
  rule path list to take effect at the edge.)

Tests: test_brief_cache_boundary. 228 pytest + 11 vitest.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/routes/admin/+page.svelte

@@ -28,12 +28,22 @@
       return;
     }
     try {
-      await loadStats();
-      feedback = await getJSON('/api/admin/feedback');
-      candidates = await getJSON('/api/admin/candidates');
-      wpPool = await getJSON('/api/admin/word/pool');
-      clientErrors = await getJSON('/api/admin/client-errors');
-      wsThemes = await getJSON('/api/admin/wordsearch/themes');
+      // Load all panels in PARALLEL — these were sequential awaits, so the page
+      // paid the (uncached, origin) round-trip six times back-to-back. One batch
+      // instead of a chain.
+      const [, fb, cand, wp, ce, ws] = await Promise.all([
+        loadStats(),
+        getJSON('/api/admin/feedback'),
+        getJSON('/api/admin/candidates'),
+        getJSON('/api/admin/word/pool'),
+        getJSON('/api/admin/client-errors'),
+        getJSON('/api/admin/wordsearch/themes'),
+      ]);
+      feedback = fb;
+      candidates = cand;
+      wpPool = wp;
+      clientErrors = ce;
+      wsThemes = ws;
     } catch {
       error = "Couldn't load stats.";
     }