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.

lib/maptool/livetrack_store.rb

@@ -55,6 +55,31 @@ module Maptool
       end
     end
 
+    # Ended sessions (summary["active"] == false) that ended more than
+    # `stale_after` seconds ago and aren't already flagged unavailable —
+    # i.e. ones Garmin has very likely stopped serving the live-track page
+    # for, but that haven't been confirmed yet (see app.rb's startup
+    # check). Excludes already-flagged ones so that check doesn't re-hit
+    # Garmin, on every restart, for a session it already told us is gone.
+    def stale_ended(stale_after)
+      cutoff = Time.now.utc - stale_after
+      list.select { |s| stale_ended?(s, cutoff) }
+    end
+
+    # Flags a session's live-track page as no longer served by Garmin, once
+    # confirmed (see app.rb's startup check) — the summary itself (small,
+    # and a record of the activity, see the class comment) is left alone.
+    def mark_unavailable(session_id)
+      @mutex.synchronize do
+        sessions = read
+        session = sessions.find { |s| s["id"] == session_id }
+        return unless session
+
+        session["map_available"] = false
+        write(sessions)
+      end
+    end
+
     # Most recently received first.
     def list
       prune(read)
@@ -77,6 +102,20 @@ module Maptool
       Time.now.utc # malformed/missing timestamp: don't let it block pruning
     end
 
+    def stale_ended?(session, cutoff)
+      summary = session["summary"]
+      return false unless summary && summary["active"] == false && session["map_available"] != false
+
+      ended = ended_at(summary)
+      ended && ended < cutoff
+    end
+
+    def ended_at(summary)
+      Time.parse(summary["end"])
+    rescue ArgumentError, TypeError
+      nil # malformed/missing "end": can't tell how long it's been, so don't guess
+    end
+
     def write(sessions)
       File.write(@path, JSON.pretty_generate(sessions))
     end