fix(admin): answer 404, not 500, when two restores race (#281)

restore-original's precheck (existing.original_image_path === null) and restoreImageOriginal's own guard (WHERE ... AND original_image_path IS NOT NULL) could disagree under a race: two concurrent restores, or a rapid double-click, could both pass the precheck before either commits, and the loser's UPDATE would then match zero rows and throw. The handler had no try/catch around that call, so the throw propagated through asyncRoute to the app-level error handler and the caller got a bare 500, breaking the route's documented 200 | 404 contract even though the row itself was left correct.

Wraps the restoreImageOriginal call in a try/catch, matching the shape remove-background already uses in this file, but answering 404 rather than 502: losing this race means another admin already finished the restore, not that a downstream service failed. Adds a comment on the catch explaining why it exists, and a test that fires two restores concurrently and asserts neither comes back 500.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/src/routes/adminItemDrafts.ts

@@ -300,7 +300,20 @@ router.post(
       return res.status(404).json({ error: 'this photo has no original to restore' });
     }
 
-    await restoreImageOriginal(Number(imageId));
+    try {
+      await restoreImageOriginal(Number(imageId));
+    } catch (err) {
+      // This precheck and restoreImageOriginal's own `original_image_path IS
+      // NOT NULL` guard can disagree under a race: two concurrent restores (or
+      // a double-click) can both pass the precheck before either commits, and
+      // the loser's UPDATE then matches zero rows and throws. That is not a
+      // sign anything is wrong — it means another request already did the
+      // restore, so the honest answer is the same 404 the precheck itself
+      // gives, not the generic 500 an uncaught throw would produce here.
+      console.error(`[drafts] restore for image ${imageId}:`, err);
+      return res.status(404).json({ error: 'this photo has no original to restore' });
+    }
+
     res.json(await imageOfItem(itemId, imageId));
   })
 );