Flag LiveTrack sessions whose map Garmin no longer serves

Ended sessions past the ~24h Garmin appears to keep a live-track page
around for get verified once at app startup and, once confirmed dead,
flagged so the panel greys them out instead of only failing after a
click. Already-flagged sessions are skipped on later startups so this
doesn't keep re-hitting Garmin for the same dead session.

app.rb

@@ -325,6 +325,25 @@ module Maptool
       json sessions: LIVETRACK_STORE.list
     end
 
+    # Garmin stops serving a session's live-track page some while after the
+    # activity ends (its own session/token URL, not the API — see
+    # LiveTrackClient) — the exact cutoff isn't documented, ~24h is what's
+    # been observed. Ended sessions old enough to likely be past that are
+    # verified once at startup (LiveTrackStore#stale_ended already skips
+    # ones already flagged, so this doesn't re-hit Garmin for the same
+    # dead session on every restart), so the panel can grey them out up
+    # front instead of only failing after a user clicks one and waits on
+    # a fetch.
+    LIVETRACK_MAP_STALE_AFTER = 24 * 60 * 60 # seconds
+
+    Thread.new do
+      LIVETRACK_STORE.stale_ended(LIVETRACK_MAP_STALE_AFTER).each do |session|
+        LiveTrackClient.fetch(session["id"], session["token"])
+      rescue LiveTrackClient::FetchError
+        LIVETRACK_STORE.mark_unavailable(session["id"])
+      end
+    end
+
     # Garmin's own API (livetrack.garmin.com/api/v2/...) 403s a plain server
     # request no matter what headers it gets — see LiveTrackClient — so this
     # fetches the session page itself and pulls the track out of that.