fix: sibling next/prev ordering for duplicate numberSort

Backgound

`/api/v1/books/{bookId}/next` used only `number_sort` for ordering and keyset seek.
When multiple books shared the same `number_sort`, the next/previous book could be unstable and diverge from `On Deck` selection, which uses number_sort + book_id as a tie-breaker.

Reason

`findSiblingSeries` ordered and seeked solely by number_sort, leaving ties without deterministic ordering.

Fix

Align sibling ordering with `On Deck` by adding `book_id` as a secondary sort key and seek parameter. This keeps endpoint semantics while making ties stable.

Tests

Add a sibling test with duplicate numberSort to assert stable next/previous results.

komga/src/main/kotlin/org/gotson/komga/infrastructure/jooq/main/BookDtoDao.kt

@@ -323,11 +323,18 @@ class BookDtoDao(
     val seriesId = record.get(0, String::class.java)
     val numberSort = record.get(1, Float::class.java)
 
+    val orderBy =
+      if (next) {
+        listOf(d.NUMBER_SORT.asc(), b.ID.asc())
+      } else {
+        listOf(d.NUMBER_SORT.desc(), b.ID.desc())
+      }
+
     return dslRO
       .selectBase(userId)
       .where(b.SERIES_ID.eq(seriesId))
-      .orderBy(d.NUMBER_SORT.let { if (next) it.asc() else it.desc() })
-      .seek(numberSort)
+      .orderBy(orderBy)
+      .seek(numberSort, bookId)
       .limit(1)
       .fetchAndMap(dslRO)
       .firstOrNull()

komga/src/test/kotlin/org/gotson/komga/interfaces/api/rest/BookControllerTest.kt

@@ -727,6 +727,50 @@ class BookControllerTest(
         .get("/api/v1/books/${book3.id}/next")
         .andExpect { status { isNotFound() } }
     }
+
+    @Test
+    @WithMockCustomUser
+    fun `given series with duplicate numberSort when getting siblings then order is stable`() {
+      val book1 = makeBook("1", libraryId = library.id, id = "1")
+      val book2 = makeBook("2", libraryId = library.id, id = "2")
+      val book3 = makeBook("3", libraryId = library.id, id = "3")
+      makeSeries(name = "series", libraryId = library.id).let { series ->
+        seriesLifecycle.createSeries(series).also { created ->
+          val books = listOf(book1, book2, book3)
+          seriesLifecycle.addBooks(created, books)
+          seriesLifecycle.sortBooks(created)
+        }
+      }
+
+      bookMetadataRepository.findById(book1.id).let { metadata ->
+        bookMetadataRepository.update(metadata.copy(numberSort = 1F))
+      }
+      bookMetadataRepository.findById(book2.id).let { metadata ->
+        bookMetadataRepository.update(metadata.copy(numberSort = 1F))
+      }
+      bookMetadataRepository.findById(book3.id).let { metadata ->
+        bookMetadataRepository.update(metadata.copy(numberSort = 2F))
+      }
+
+      mockMvc
+        .get("/api/v1/books/${book1.id}/next")
+        .andExpect {
+          status { isOk() }
+          jsonPath("$.name") { value("2") }
+        }
+      mockMvc
+        .get("/api/v1/books/${book2.id}/previous")
+        .andExpect {
+          status { isOk() }
+          jsonPath("$.name") { value("1") }
+        }
+      mockMvc
+        .get("/api/v1/books/${book2.id}/next")
+        .andExpect {
+          status { isOk() }
+          jsonPath("$.name") { value("3") }
+        }
+    }
   }
 
   @Nested