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>
