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.

src/test/java/core_service/modules/library/api/LibraryControllerSteamIdGoneTest.java

@@ -0,0 +1,62 @@
+package core_service.modules.library.api;
+
+import core_service.modules.library.repo.UserSteamSyncRepository;
+import core_service.shared.test.AbstractIntegrationTest;
+import io.jsonwebtoken.Jwts;
+import jakarta.servlet.http.Cookie;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.webmvc.test.autoconfigure.AutoConfigureMockMvc;
+import org.springframework.jdbc.core.JdbcTemplate;
+import org.springframework.test.web.servlet.MockMvc;
+
+import java.time.Duration;
+import java.time.Instant;
+import java.util.Date;
+
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;
+
+/**
+ * A user who unlinked the Steam ID, or lost it to a verified link, must drop out of the nightly sync.
+ */
+@AutoConfigureMockMvc
+class LibraryControllerSteamIdGoneTest extends AbstractIntegrationTest {
+
+    private static final long USER_ID = 77L;
+
+    @Autowired
+    private MockMvc mockMvc;
+    @Autowired
+    private JdbcTemplate jdbc;
+    @Autowired
+    private UserSteamSyncRepository userSteamSyncRepository;
+
+    @BeforeEach
+    void seedSyncedUser() {
+        jdbc.update("DELETE FROM user_steam_sync WHERE user_id = ?", USER_ID);
+        jdbc.update("INSERT INTO user_steam_sync (user_id, steam_id, last_synced_at, last_sync_status, user_seen_at) "
+                + "VALUES (?, '76561198000000001', now(), 'OK', now())", USER_ID);
+    }
+
+    private static Cookie loginWithoutSteamId() {
+        return new Cookie("tbp_access", Jwts.builder()
+                .subject("unlinked@example.com")
+                .claim("userId", String.valueOf(USER_ID))
+                .expiration(new Date(System.currentTimeMillis() + 60_000))
+                .signWith(TEST_KEYS.getPrivate(), Jwts.SIG.EdDSA)
+                .compact());
+    }
+
+    @Test
+    void gamesCount_WithoutSteamIdClaim_StopsTheNightlySyncOfTheOldId() throws Exception {
+        mockMvc.perform(get("/api/library/games-count").cookie(loginWithoutSteamId())).andExpect(status().isOk());
+
+        assertTrue(userSteamSyncRepository.findByUserSeenAtAfter(Instant.now().minus(Duration.ofDays(14))).stream()
+                .noneMatch(sync -> sync.getUserId() == USER_ID));
+        // The old ID stays: a later link with another ID must still soft-delete the old games.
+        assertTrue(userSteamSyncRepository.findById(USER_ID).orElseThrow().getSteamId().equals("76561198000000001"));
+    }
+}