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>

Jarvis/Store/StoryStore.swift

@@ -312,7 +312,8 @@ final class StoryStore: ObservableObject {
             consensus: old.consensus,
             conflict: old.conflict,
             updatedAt: old.updatedAt,
-            createdAt: old.createdAt
+            createdAt: old.createdAt,
+            firstSeenAt: old.firstSeenAt
         )
         stories[idx] = updated
         stories.sort(by: StorySummary.feedOrder)