fix: prevent duplicate item creation with barcode conflict detection

Backend:
- Added UNIQUE constraint check on barcode before item creation
- Returns 409 Conflict with user-friendly message if duplicate exists
- Prevents sqlite3.IntegrityError crashes

Frontend:
- Improved error handling for cloud sync failures
- Detects 409 status code (duplicate barcode)
- Shows specific error message to user
- Gracefully falls back to local-only save

Example error message: 'Item already exists. Item with barcode P66093-002 already exists (ID: 42). Update it instead or use a different barcode.'

backend/routers/items.py

@@ -97,6 +97,15 @@ def create_item(
     current_user: auth.TokenData = Depends(auth.get_current_user)
 ):
     """[C-01] Create item — only for authenticated users. [M-02] user_id from token."""
+    # [DUPLICATE CHECK] Prevent duplicate barcodes (part numbers)
+    if item.barcode:
+        existing = db.query(models.Item).filter(models.Item.barcode == item.barcode).first()
+        if existing:
+            raise HTTPException(
+                status_code=409,
+                detail=f"Item with barcode '{item.barcode}' already exists (ID: {existing.id}). Update it instead or use a different barcode."
+            )
+
     # [AUTO-PERSIST] Create Category/Color if not exists
     if item.category:
         cat = db.query(models.Category).filter(models.Category.name == item.category).first()

frontend/app/page.tsx

@@ -170,9 +170,16 @@ export default function Home() {
           await inventoryApi.createItem(currentUser.id, itemData);
           toast.success("Item saved to cloud catalog!");
         } catch (err: any) {
-          console.error("Cloud sync failed:", err.message);
+          const status = err.response?.status;
+          const detail = err.response?.data?.detail || err.message;
+          console.error("Cloud sync failed:", detail);
+
+          if (status === 409) {
+            toast.error(`Item already exists. ${detail}`);
+          } else {
             toast.error("Item saved locally. Cloud sync failed.");
           }
+        }
       } else if (!isOnline) {
         toast.success("Item saved locally. Will sync when online.");
       } else {