fix: interleave uploaded chapters by number in the chapter list

Uploaded chapters have no source_index, and ORDER BY source_index DESC NULLS
LAST sorted every one of them after all crawled chapters regardless of number
— an uploaded chapter 5 landed after crawled chapter 100. Slot each uploaded
chapter by number, just before the crawled chapter with the smallest greater
number, so it interleaves. Crawled rows keep their reversed source-DOM order,
preserving the deliberate placement of non-numeric entries (migration 0021).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/tests/api_chapters.rs

@@ -148,6 +148,49 @@ async fn list_chapters_returned_in_number_order(pool: PgPool) {
     assert_eq!(body["items"][1]["title"], serde_json::Value::Null);
 }
 
+#[sqlx::test(migrations = "./migrations")]
+async fn list_interleaves_uploaded_and_crawled_chapters_by_number(pool: PgPool) {
+    // Mixed source: crawled chapters carry a `source_index`; an uploaded
+    // chapter has none. The uploaded chapter must interleave by NUMBER, not sort
+    // after every crawled chapter (the old `source_index DESC NULLS LAST` bug
+    // dumped uploaded chapter 2 to the end, giving [1, 3, 2]).
+    let h = common::harness(pool.clone());
+    let (_, cookie) = common::register_user(&h.app).await;
+    let manga_id = seed_manga(&h, &cookie, "Berserk").await;
+
+    // Two crawled chapters (1 and 3) with DOM positions — newest-first, so
+    // chapter 3 is at source_index 0 and chapter 1 at source_index 1.
+    let c1 = seed_chapter(&pool, manga_id, 1, Some("Crawled One")).await;
+    let c3 = seed_chapter(&pool, manga_id, 3, Some("Crawled Three")).await;
+    sqlx::query("UPDATE chapters SET source_index = 1 WHERE id = $1")
+        .bind(c1)
+        .execute(&pool)
+        .await
+        .unwrap();
+    sqlx::query("UPDATE chapters SET source_index = 0 WHERE id = $1")
+        .bind(c3)
+        .execute(&pool)
+        .await
+        .unwrap();
+    // Uploaded chapter 2 — no source_index.
+    seed_chapter(&pool, manga_id, 2, Some("Uploaded Two")).await;
+
+    let resp = h
+        .app
+        .oneshot(common::get(&format!("/api/v1/mangas/{manga_id}/chapters")))
+        .await
+        .unwrap();
+    assert_eq!(resp.status(), StatusCode::OK);
+    let body = common::body_json(resp).await;
+    let numbers: Vec<i64> = body["items"]
+        .as_array()
+        .unwrap()
+        .iter()
+        .map(|c| c["number"].as_i64().unwrap())
+        .collect();
+    assert_eq!(numbers, vec![1, 2, 3], "uploaded chapter 2 must slot between 1 and 3");
+}
+
 #[sqlx::test(migrations = "./migrations")]
 async fn list_chapters_returns_404_for_unknown_manga(pool: PgPool) {
     let h = common::harness(pool);