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/api.py

@@ -159,6 +159,7 @@ class FeedResponse(BaseModel):
 class BriefResponse(BaseModel):
     brief_date: str | None
     title: str | None
+    generated_at: str | None = None  # freshness stamp: changes only when content changes
     items: list[Article]
 
 
@@ -336,6 +337,7 @@ def create_app() -> FastAPI:
         return BriefResponse(
             brief_date=data["brief_date"],
             title=data["title"],
+            generated_at=data.get("created_at"),
             items=[Article.from_row(r) for r in items],
         )