fix(admin): three review findings — audit no-op, 404, chapter priority (0.41.1)

- admin_safe_set_is_admin: short-circuit when target.is_admin == value,
  before writing audit. PATCH {is_admin: true} on someone already admin
  previously wrote a misleading "promote_user" row even though the UPDATE
  was a no-op.

- list_chapters (/admin/mangas/:id/chapters): explicit exists() check on
  manga_id, returns 404 instead of 200 [] for a typo'd / deleted manga.

- ChapterSyncState priority: the Failed branch now requires page_count = 0,
  so a chapter with pages on disk AND a historical dead job (from a
  re-download attempt that crashed) stays Synced. The old order
  contradicted Synced's documented "downloaded at some point" contract.
  Doc comments updated alongside the SQL.

Three new regression tests pin the behaviour.

backend/tests/api_admin_users.rs

@@ -330,6 +330,37 @@ async fn promote_writes_audit_row(pool: PgPool) {
     assert_eq!(target, Some(b.id));
 }
 
+#[sqlx::test(migrations = "./migrations")]
+async fn redundant_promote_does_not_write_audit_row(pool: PgPool) {
+    // Regression: PATCH {is_admin: true} on someone already admin used
+    // to UPDATE (no-op) and still INSERT a misleading "promote_user"
+    // audit row. Should short-circuit without touching admin_audit.
+    let h = common::harness(pool.clone());
+    let (_a_name, a_cookie, _a_id) = seed_admin(&pool, &h.app).await;
+    let (b_name, _b_cookie, _b_id) = seed_admin(&pool, &h.app).await; // already admin
+
+    let b = repo::user::find_by_username(&pool, &b_name)
+        .await
+        .unwrap()
+        .unwrap();
+    let resp = h
+        .app
+        .oneshot(common::patch_json_with_cookie(
+            &format!("/api/v1/admin/users/{}", b.id),
+            json!({ "is_admin": true }),
+            &a_cookie,
+        ))
+        .await
+        .unwrap();
+    assert_eq!(resp.status(), StatusCode::OK);
+
+    let (count,): (i64,) = sqlx::query_as("SELECT COUNT(*) FROM admin_audit")
+        .fetch_one(&pool)
+        .await
+        .unwrap();
+    assert_eq!(count, 0, "no-op promote must not write audit row");
+}
+
 #[sqlx::test(migrations = "./migrations")]
 async fn delete_writes_audit_row(pool: PgPool) {
     let h = common::harness(pool.clone());