fix: make bookmark add idempotent instead of 409

Re-adding an already-bookmarked manga returned 409, which the UI surfaced as
a false "Could not add bookmark" toast — collections are idempotent, bookmarks
weren't. On a unique violation, fetch and return the existing bookmark, and
answer 200 (vs 201 for a fresh insert). The frontend toggle already treats a
successful POST as done, so the false toast disappears with no client change.

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

backend/tests/api_bookmarks.rs

@@ -52,7 +52,7 @@ async fn create_then_list_returns_only_own(pool: PgPool) {
 }
 
 #[sqlx::test(migrations = "./migrations")]
-async fn create_returns_409_on_duplicate_manga_level(pool: PgPool) {
+async fn create_is_idempotent_on_duplicate_manga_level(pool: PgPool) {
     let h = common::harness(pool);
     let (_, cookie) = common::register_user(&h.app).await;
     let manga_id = common::seed_manga_via_api(&h.app, &cookie, "Berserk").await;
@@ -64,12 +64,26 @@ async fn create_returns_409_on_duplicate_manga_level(pool: PgPool) {
             &cookie,
         )
     };
+    // First add creates (201); re-adding is an idempotent no-op (200) that
+    // returns the SAME bookmark rather than a 409 — collections behave this way
+    // too, so the UI shouldn't surface a false "Could not add bookmark" error.
     let first = h.app.clone().oneshot(make()).await.unwrap();
     assert_eq!(first.status(), StatusCode::CREATED);
-    let second = h.app.oneshot(make()).await.unwrap();
-    assert_eq!(second.status(), StatusCode::CONFLICT);
-    let body = common::body_json(second).await;
-    assert_eq!(body["error"]["code"], "conflict");
+    let first_body = common::body_json(first).await;
+
+    let second = h.app.clone().oneshot(make()).await.unwrap();
+    assert_eq!(second.status(), StatusCode::OK);
+    let second_body = common::body_json(second).await;
+    assert_eq!(second_body["id"], first_body["id"], "same bookmark returned");
+
+    // Exactly one row exists.
+    let list = h
+        .app
+        .oneshot(common::get_with_cookie("/api/v1/me/bookmarks", &cookie))
+        .await
+        .unwrap();
+    let body = common::body_json(list).await;
+    assert_eq!(body["items"].as_array().unwrap().len(), 1);
 }
 
 #[sqlx::test(migrations = "./migrations")]
@@ -225,13 +239,16 @@ async fn concurrent_manga_bookmarks_serialised_by_unique_index(pool: PgPool) {
 
     let (s1, s2) = tokio::join!(f1, f2);
     let statuses = [s1.unwrap(), s2.unwrap()];
+    // The unique index serialises the two inserts: one wins with 201, the other
+    // hits the violation and — now idempotent — returns the existing bookmark
+    // with 200 rather than a 409.
     assert!(
         statuses.contains(&StatusCode::CREATED),
         "expected one winner with 201, got {statuses:?}"
     );
     assert!(
-        statuses.contains(&StatusCode::CONFLICT),
-        "expected one loser with 409 (the partial unique index), got {statuses:?}"
+        statuses.contains(&StatusCode::OK),
+        "expected the loser to return 200 (idempotent), got {statuses:?}"
     );
 }