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/Models/Models.swift

@@ -61,6 +61,11 @@ struct StorySummary: Codable, Identifiable, Hashable {
     let conflict: String?
     let updatedAt: Date
     let createdAt: Date
+    /// When the cluster first appeared. `updatedAt` bumps on every article the
+    /// backend attaches to the cluster (even a loosely-related one), so it can
+    /// read "1 hr ago" on stories that are actually weeks old — `firstSeenAt`
+    /// is the honest age. Optional so decoding tolerates older API responses.
+    let firstSeenAt: Date?
 }
 
 struct ScoreBreakdown: Codable, Hashable {
@@ -185,6 +190,10 @@ final class CachedStory {
     /// Original publish timestamp. Defaults to updatedAt for rows cached before
     /// this field was added — SwiftData handles the automatic migration.
     var createdAt: Date = Date()
+    /// When the cluster first appeared — the honest age, unlike `updatedAt` which
+    /// bumps whenever the backend attaches another article. Defaults to `updatedAt`
+    /// for rows cached before this field was added.
+    var firstSeenAt: Date = Date()
     var cachedAt: Date
 
     init(from summary: StorySummary) {
@@ -199,6 +208,7 @@ final class CachedStory {
         self.conflict = summary.conflict
         self.updatedAt = summary.updatedAt
         self.createdAt = summary.createdAt
+        self.firstSeenAt = summary.firstSeenAt ?? summary.updatedAt
         self.cachedAt = Date()
     }
 }