feat(vegas): keep live games in the ticker by default

display.vegas_scroll.live_in_ticker now defaults to true: through a live
game the marquee keeps running and the live scoreboard takes extra turns in
it -- its cards updating in place while they scroll -- instead of the ticker
giving way to the full-screen scoreboard.

The new default would reach nobody on its own: every existing config holds
an explicit false copied from the template (there was no control for it),
and the template merge only adds missing keys. ConfigManager therefore turns
a stored false on once, with a backup, and records live_in_ticker_migrated
so a false chosen afterwards stays. The marker is never in the template.

A "Keep live games in the ticker" checkbox under Vegas mode sets it. Tests
that pin the full-screen takeover now say live_in_ticker=false.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/display_controller.py

@@ -2206,7 +2206,7 @@ class DisplayController:
         """Whether live content should stay in the ticker instead of preempting it."""
         coordinator = self.vegas_coordinator
         config = getattr(coordinator, 'vegas_config', None)
-        return bool(getattr(config, 'live_in_ticker', False))
+        return bool(getattr(config, 'live_in_ticker', True))
 
     def _check_live_priority(self, advance=False):
         """Return the live-priority mode to display, or None if nothing is live.