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/Views/Home/SignalFeedView.swift

@@ -481,7 +481,7 @@ struct SignalFeedView: View {
                 if !story.sources.isEmpty {
                     SourceChips(names: story.sources.map(\.name), total: story.sourceCount)
                 }
-                Text("\(story.sourceCount) sources · \(story.updatedAt.timeAgoShort()) ago")
+                Text("\(story.sourceCount) sources · \((story.firstSeenAt ?? story.updatedAt).timeAgoShort()) ago")
                     .font(.system(size: 11, weight: .regular, design: .monospaced))
                     .foregroundStyle(Palette.tertiaryText)
             }
@@ -768,7 +768,8 @@ extension StorySummary {
             consensus: cached.consensus,
             conflict: cached.conflict,
             updatedAt: cached.updatedAt,
-            createdAt: cached.createdAt
+            createdAt: cached.createdAt,
+            firstSeenAt: cached.firstSeenAt
         )
     }
 }