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/database/order.py

@@ -24,6 +24,10 @@ from sqlalchemy import (
     String,
     Text,
 )
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from models.database.order_item_exception import OrderItemException
 from sqlalchemy.dialects.sqlite import JSON
 from sqlalchemy.orm import relationship
 
@@ -201,9 +205,19 @@ class OrderItem(Base, TimestampMixin):
     inventory_reserved = Column(Boolean, default=False)
     inventory_fulfilled = Column(Boolean, default=False)
 
+    # === Exception Tracking ===
+    # True if product was not found by GTIN during import (linked to placeholder)
+    needs_product_match = Column(Boolean, default=False, index=True)
+
     # === Relationships ===
     order = relationship("Order", back_populates="items")
     product = relationship("Product")
+    exception = relationship(
+        "OrderItemException",
+        back_populates="order_item",
+        uselist=False,
+        cascade="all, delete-orphan",
+    )
 
     def __repr__(self):
         return f"<OrderItem(id={self.id}, order_id={self.order_id}, product_id={self.product_id}, gtin='{self.gtin}')>"
@@ -222,3 +236,10 @@ class OrderItem(Base, TimestampMixin):
     def is_declined(self) -> bool:
         """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.blocks_confirmation