feat(library): sort by rating and priority with a stable order

Games without a rating or priority come last in both directions. Priority
sorts HIGH, MEDIUM, LOW instead of by enum name, and appid breaks every tie,
so pages no longer repeat or skip games with equal values.

src/test/java/core_service/modules/library/repo/UserGameRepositoryTest.java

@@ -1,5 +1,6 @@
 package core_service.modules.library.repo;
 
+import core_service.modules.library.domain.GamePriority;
 import core_service.modules.library.domain.GameStatus;
 import core_service.modules.library.domain.StatusCount;
 import core_service.modules.library.domain.SteamGame;
@@ -95,6 +96,51 @@ class UserGameRepositoryTest extends AbstractPersistenceTest {
         assertEquals(2, page.getTotalElements());
     }
 
+    private List<Long> sortedAppids(Sort sort) {
+        return userGameRepository.findOwnedGames(USER_ID, null, null, PageRequest.of(0, 20, UserGameRepository.querySort(sort)))
+                .map(UserGame::getAppid).getContent();
+    }
+
+    @Test
+    void querySort_RatingDesc_PutsUnratedLastAndBreaksTiesByAppid() {
+        setBacklog(220L, 3, null);
+        setBacklog(620L, 5, null);
+        setBacklog(700L, 3, null);
+
+        assertEquals(List.of(620L, 220L, 700L, 380L), sortedAppids(Sort.by(Sort.Direction.DESC, "rating")));
+    }
+
+    @Test
+    void querySort_PriorityDesc_IsHighMediumLowThenNone() {
+        setBacklog(220L, null, GamePriority.LOW);
+        setBacklog(380L, null, GamePriority.HIGH);
+        setBacklog(700L, null, GamePriority.MEDIUM);
+
+        assertEquals(List.of(380L, 700L, 220L, 620L), sortedAppids(Sort.by(Sort.Direction.DESC, "priority")));
+    }
+
+    @Test
+    void querySort_EqualPlaytime_BreaksTiesByAppid() {
+        assertEquals(List.of(620L, 220L, 380L, 700L), sortedAppids(Sort.by(Sort.Direction.DESC, "playtimeMinutes")));
+    }
+
+    // unrated LAST (nullsLast) and the querySort javadoc promises "then none" — one API, two opposite null rules.
+    @Test
+    void querySort_PriorityAsc_PutsNoneLast() {
+        setBacklog(220L, null, GamePriority.LOW);
+        setBacklog(380L, null, GamePriority.HIGH);
+        setBacklog(700L, null, GamePriority.MEDIUM);
+
+        assertEquals(List.of(220L, 700L, 380L, 620L), sortedAppids(Sort.by(Sort.Direction.ASC, "priority")));
+    }
+
+    private void setBacklog(long appid, Integer rating, GamePriority priority) {
+        UserGame game = userGameRepository.findById(new UserGameId(USER_ID, appid)).orElseThrow();
+        game.setRating(rating);
+        game.setPriority(priority);
+        userGameRepository.flush();
+    }
+
     @Test
     void containsPattern_BlankIsNoFilter() {
         assertNull(UserGameRepository.containsPattern("  "));