fix(sync): stop the nightly sync when the Steam ID is gone

After an unlink in Auth, or when a verified link took the ID away, the
nightly job kept syncing the old Steam ID for up to 14 days. A
games-count request without a steamId claim now clears user_seen_at.
steam_id stays, so a later link with another ID still soft-deletes the
old games.

readme.md

@@ -129,7 +129,8 @@ Inside a module: `api` (controllers, DTOs), `service` (business logic), `domain`
 1. `GET /api/library/games`, `/games-count` and `/picker` call `LibrarySyncService.syncIfStale`. It starts a sync
    when the last one is older than 24 hours, older than 15 minutes after a failure, or was for another Steam ID.
    `POST /api/library/sync` starts one at once, but at most every 60 seconds (else `429` with `Retry-After`).
-2. A nightly job (23:55, Europe/Berlin) syncs every account used in the last 14 days, one after the other.
+2. A nightly job (23:55, Europe/Berlin) syncs every account used in the last 14 days, one after the other. A
+   request without a `steamId` claim (the user unlinked the Steam account in Auth) takes the user out of it.
 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