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/src/api/bookmarks.rs

@@ -72,7 +72,7 @@ async fn create(
         }
     }
 
-    let bookmark = repo::bookmark::create(
+    let (bookmark, created) = repo::bookmark::create(
         &state.db,
         user.id,
         input.manga_id,
@@ -103,7 +103,13 @@ async fn create(
         }
     });
 
-    Ok((StatusCode::CREATED, Json(bookmark)))
+    // 201 for a fresh bookmark, 200 when it already existed (idempotent add).
+    let status = if created {
+        StatusCode::CREATED
+    } else {
+        StatusCode::OK
+    };
+    Ok((status, Json(bookmark)))
 }
 
 async fn delete_one(