fix(sync): keep old playtime off the day a game shows up

A game the sync saw for the first time put its whole playtime on that
day (40 h "longest gaming day" after skip_unvetted_apps=0). Growth now
counts at most Steam's last two weeks, and one game gets at most 24 h
per day. V18 drops the bad rows and caps old ones.

readme.md

@@ -169,7 +169,8 @@ Inside a module: `api` (controllers, DTOs), `service` (business logic), `domain`
 3. `LibrarySyncWorker` runs in the background. `user_steam_sync` lets only one run per user start.
 4. The worker loads the wishlist and the owned games from Steam. It updates `steam_game` and `user_game`, adds
    the playtime growth of the day to `user_game_playtime_day`, and soft-deletes Steam games the user no longer
-   owns. A BACKLOG game with new playtime becomes PLAYING, unless the user chose its status. When Steam returns no
+   owns. The growth counts at most Steam's playtime of the last two weeks, so a game seen for the first time brings
+   no old playtime, and one game gets at most 24 hours per day. A BACKLOG game with new playtime becomes PLAYING, unless the user chose its status. When Steam returns no
    games, the library stays as it is.
 5. `TimeToBeatService` and `MoodService` ask IGDB for missing or old time-to-beat, mood and genre data, for Steam
    games and manual games. Hidden games only get their playtime updated.