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>

goodnews/briefs.py

@@ -14,10 +14,29 @@ def build_daily_brief(
     window_days: int = 3,
 ) -> int:
     target_date = brief_date or date.today().isoformat()
+
+    # Compose the selection first so we can tell whether anything actually
+    # changed. A calm daily brief shouldn't repeatedly hand the reader a locked
+    # door: push paywalled candidates below readable ones (stable sort) first.
+    rows = _candidate_articles(conn, target_date, window_days)
+    rows = sorted(rows, key=lambda r: is_paywalled(r["canonical_url"]))
+    selected = _select_diverse(rows, limit)
+    selected_ids = [row["id"] for row in selected]
+
     existing = conn.execute("SELECT id FROM daily_briefs WHERE brief_date = ?", (target_date,)).fetchone()
-    if existing and not replace:
-        return int(existing["id"])
-    if existing and replace:
+    if existing:
+        existing_ids = [
+            r["article_id"]
+            for r in conn.execute(
+                "SELECT article_id FROM daily_brief_items WHERE brief_id = ? ORDER BY rank",
+                (existing["id"],),
+            )
+        ]
+        # Idempotent: if the selection is unchanged, leave the brief (and its
+        # created_at freshness stamp) alone — a 15-minute rebuild with no new
+        # data is a no-op, so a reader's pinned view holds.
+        if existing_ids == selected_ids or not replace:
+            return int(existing["id"])
         conn.execute("DELETE FROM daily_briefs WHERE id = ?", (existing["id"],))
 
     brief_id = conn.execute(
@@ -25,12 +44,6 @@ def build_daily_brief(
         (target_date, f"Highlights from Today - {target_date}"),
     ).lastrowid
 
-    rows = _candidate_articles(conn, target_date, window_days)
-    # A calm daily brief shouldn't repeatedly hand the reader a locked door:
-    # push paywalled candidates below readable ones (stable, so composite order
-    # is preserved within each group) before selecting the five.
-    rows = sorted(rows, key=lambda r: is_paywalled(r["canonical_url"]))
-    selected = _select_diverse(rows, limit)
     for index, row in enumerate(selected, start=1):
         conn.execute(
             """