fix(library): keep backlog edits made during a sync, reject fractional ratings

UserGame now uses @DynamicUpdate, so the sync flush writes only the columns it
changed and no longer overwrites a backlog update committed meanwhile.

src/main/resources/application.yml

@@ -3,6 +3,9 @@ spring:
     name: tbp-core-service
   profiles:
     active: prod
+  jackson:
+    deserialization:
+      accept-float-as-int: false
   jpa:
     open-in-view: true
   flyway: