feat: add order item exception system for graceful product matching

Replaces the "fail on missing product" behavior with graceful handling:
- Orders import even when products aren't found by GTIN
- Unmatched items link to a per-vendor placeholder product
- Exceptions tracked in order_item_exceptions table for QC resolution
- Order confirmation blocked until exceptions are resolved
- Auto-matching when products are imported via catalog sync

New files:
- OrderItemException model and migration
- OrderItemExceptionService with CRUD and resolution logic
- Admin and vendor API endpoints for exception management
- Domain exceptions for error handling

Modified:
- OrderItem: added needs_product_match flag and exception relationship
- OrderService: graceful handling with placeholder products
- MarketplaceProductService: auto-match on product import
- Letzshop confirm endpoints: blocking check for unresolved exceptions

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

models/schema/order.py

@@ -57,6 +57,19 @@ class OrderItemCreate(BaseModel):
     quantity: int = Field(..., ge=1)
 
 
+class OrderItemExceptionBrief(BaseModel):
+    """Brief exception info for embedding in order item responses."""
+
+    model_config = ConfigDict(from_attributes=True)
+
+    id: int
+    original_gtin: str | None
+    original_product_name: str | None
+    exception_type: str
+    status: str
+    resolved_product_id: int | None
+
+
 class OrderItemResponse(BaseModel):
     """Schema for order item response."""
 
@@ -84,6 +97,10 @@ class OrderItemResponse(BaseModel):
     inventory_reserved: bool
     inventory_fulfilled: bool
 
+    # Exception tracking
+    needs_product_match: bool = False
+    exception: OrderItemExceptionBrief | None = None
+
     created_at: datetime
     updated_at: datetime
 
@@ -102,6 +119,13 @@ class OrderItemResponse(BaseModel):
         """Check if item was declined (unavailable)."""
         return self.item_state == "confirmed_unavailable"
 
+    @property
+    def has_unresolved_exception(self) -> bool:
+        """Check if item has an unresolved exception blocking confirmation."""
+        if not self.exception:
+            return False
+        return self.exception.status in ("pending", "ignored")
+
 
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 # Customer Snapshot Schemas