fix: display story age from firstSeenAt, not updatedAt

Old stories were reading "1 hr ago" because updatedAt bumps every time
the backend attaches another article to a cluster, however loosely
related. firstSeenAt is when the cluster actually first appeared and
is already in the API response — just wasn't decoded.

Verified live: story_1a37c858235b (F1 Austria GP recap) has
firstSeenAt 2026-06-16 but updatedAt 2026-07-11, freshnessScore 0.96 —
looked brand new despite being 25 days old. Also confirmed the API's
isStale field is unusable as a stopgap (always false regardless of age).

Ranking still surfaces old stories at the top — that's server-side
(signalScore/freshnessScore) and out of scope for the client per the
architecture rule against re-ranking. Logged in BACKLOG #15.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/BACKLOG.md

@@ -42,8 +42,15 @@ Status: 🔴 open · 🟡 mitigated client-side (real fix still pending) · 🟢
     Bafana exit" for the *same* `story_id`). **Fix: decay freshness from `firstSeenAt` (or
     the newest article whose relevance to the cluster core is verified), not from
     "last touched."** Tightening the TF-IDF threshold in #2 should also reduce how often
-    unrelated articles keep a stale cluster's clock alive. *No client fix possible — the
-    story object's freshness field itself is wrong before it reaches the app.*
+    unrelated articles keep a stale cluster's clock alive. Also confirmed the API's own
+    `isStale` field is unusable as a stopgap — it reads `false` on every story checked
+    regardless of actual age, so it isn't computed from a real staleness threshold.
+    *Client mitigation shipped:* age display (row meta line, hero card) now reads from
+    `firstSeenAt` instead of `updatedAt`, so the UI no longer shows "1 hr ago" on stories
+    that are actually weeks old. *Ranking is still wrong* — old stories still surface at
+    the top of the feed, because `signalScore`/`freshnessScore` (used for ordering) are
+    computed server-side and the client must not re-rank (architecture rule). The
+    real fix is still backend-side.
 
 ## Backend — features