feat(sync): start Steam sync on visit when data is older than 20 h

A visit starts a background sync when the last successful sync is
older than 20 h; one try per hour, one sync at a time. Migration V31
adds last_success_at and login_sync_tried_at. The nightly sync skips
only users with a successful sync today after 23:00. games-count
returns syncStarted.

src/main/resources/application.yml

@@ -56,7 +56,9 @@ app:
     enabled: true
     base-url: https://www.speedrun.com/api/v1
   library:
-    sync-ttl-hours: 24
+    sync-ttl-hours: 20
+    # At most one sync try per user and hour on a visit, also after a failure.
+    login-retry-minutes: 60
     force-sync-cooldown-seconds: 60
     running-stale-minutes: 5
     achievement-ttl-hours: 1