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/queries.py

@@ -120,11 +120,11 @@ def brief(conn: sqlite3.Connection, brief_date: str | None = None, limit: int =
         return {"brief_date": None, "title": None, "items": []}
 
     header = conn.execute(
-        "SELECT brief_date, title FROM daily_briefs WHERE brief_date = ?",
+        "SELECT brief_date, title, created_at FROM daily_briefs WHERE brief_date = ?",
         (target_date,),
     ).fetchone()
     if not header:
-        return {"brief_date": target_date, "title": None, "items": []}
+        return {"brief_date": target_date, "title": None, "created_at": None, "items": []}
 
     rows = conn.execute(
         f"""
@@ -143,6 +143,7 @@ def brief(conn: sqlite3.Connection, brief_date: str | None = None, limit: int =
     return {
         "brief_date": header["brief_date"],
         "title": header["title"],
+        "created_at": header["created_at"],
         "items": [dict(row) for row in rows],
     }