Fresh server data overrides a pinned brief; pin holds otherwise

Per the agreed model: the brief is server-authoritative and a client Replace is
a soft override that yields when genuinely new data arrives.
- build_daily_brief is now idempotent: if the composed selection is unchanged it
  leaves the brief (and its created_at) alone, so the timer's 15-min rebuilds are
  no-ops when no new data landed.
- /api/brief exposes generated_at (the brief's created_at = a content-change
  stamp). The client pins its view against generated_at and keeps it across plain
  refreshes, but drops it and shows the fresh server brief when generated_at
  advances. Missed stories remain in the mood feeds.

Tests: idempotent rebuild (no-op vs content change) — 93 total.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/routes/+page.svelte

@@ -76,14 +76,16 @@
     const ex = Array.from(dismissed).join(',');
     const fetched = await getJSON(`/api/brief?limit=7${q ? '&' + q : ''}${ex ? '&exclude=' + ex : ''}`);
     const view = P.loadJSON(BRIEF_VIEW_KEY, null);
-    // On a plain (re)load, keep the reader's curated view for the same day — so
-    // swaps and the hero hold steady. Re-fetch fresh on a new day's brief, or
-    // when forced (e.g. a boundary changed and must be re-applied).
-    if (!fresh && view && view.date === fetched.brief_date && Array.isArray(view.items) && view.items.length) {
-      brief = { brief_date: view.date, title: fetched.title, items: view.items };
+    // Keep the reader's curated view (swaps + hero) across plain refreshes — but
+    // only while the server's brief is unchanged. When genuinely fresh data
+    // arrives (generated_at advances), the server wins and the pin is dropped.
+    const sameServerBrief =
+      view && view.generated_at && fetched.generated_at && view.generated_at === fetched.generated_at;
+    if (!fresh && sameServerBrief && Array.isArray(view.items) && view.items.length) {
+      brief = { ...fetched, items: view.items };
     } else {
       brief = fetched;
-      P.saveJSON(BRIEF_VIEW_KEY, { date: fetched.brief_date, items: fetched.items });
+      P.saveJSON(BRIEF_VIEW_KEY, { generated_at: fetched.generated_at, items: fetched.items });
     }
     remember(brief.items);
   }
@@ -155,8 +157,9 @@
       if (i >= 0) {
         brief.items[i] = repl;
         brief = { ...brief, items: [...brief.items] };
-        // Pin the swap so this exact curated brief survives a refresh.
-        P.saveJSON(BRIEF_VIEW_KEY, { date: brief.brief_date, items: brief.items });
+        // Pin the swap against the current server brief; it holds until fresh
+        // server data arrives (then the server's version takes over).
+        P.saveJSON(BRIEF_VIEW_KEY, { generated_at: brief.generated_at, items: brief.items });
       }
     } else {
       const i = feed.findIndex((a) => a.id === article.id);