feat: add inventory transaction audit trail (Phase 2)

Adds complete audit trail for all stock movements:
- InventoryTransaction model with transaction types (reserve, fulfill,
  release, adjust, set, import, return)
- Alembic migration for inventory_transactions table
- Transaction logging in order_inventory_service for all order operations
- Captures quantity snapshots, order references, and timestamps

Each inventory operation now creates a transaction record for
accountability and debugging.

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

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

app/services/order_inventory_service.py

@@ -9,6 +9,8 @@ This service orchestrates inventory operations for orders:
 
 This is the critical link between the order and inventory systems
 that ensures stock accuracy.
+
+All operations are logged to the inventory_transactions table for audit trail.
 """
 
 import logging
@@ -22,6 +24,7 @@ from app.exceptions import (
 )
 from app.services.inventory_service import inventory_service
 from models.database.inventory import Inventory
+from models.database.inventory_transaction import InventoryTransaction, TransactionType
 from models.database.order import Order, OrderItem
 from models.schema.inventory import InventoryReserve
 
@@ -84,6 +87,51 @@ class OrderInventoryService:
         # Check if it's the placeholder product (GTIN 0000000000000)
         return order_item.product.gtin == "0000000000000"
 
+    def _log_transaction(
+        self,
+        db: Session,
+        vendor_id: int,
+        product_id: int,
+        inventory: Inventory,
+        transaction_type: TransactionType,
+        quantity_change: int,
+        order: Order,
+        reason: str | None = None,
+    ) -> InventoryTransaction:
+        """
+        Create an inventory transaction record for audit trail.
+
+        Args:
+            db: Database session
+            vendor_id: Vendor ID
+            product_id: Product ID
+            inventory: Inventory record after the operation
+            transaction_type: Type of transaction
+            quantity_change: Change in quantity (positive = add, negative = remove)
+            order: Order associated with this transaction
+            reason: Optional reason for the transaction
+
+        Returns:
+            Created InventoryTransaction
+        """
+        transaction = InventoryTransaction.create_transaction(
+            vendor_id=vendor_id,
+            product_id=product_id,
+            inventory_id=inventory.id if inventory else None,
+            transaction_type=transaction_type,
+            quantity_change=quantity_change,
+            quantity_after=inventory.quantity if inventory else 0,
+            reserved_after=inventory.reserved_quantity if inventory else 0,
+            location=inventory.location if inventory else None,
+            warehouse=inventory.warehouse if inventory else None,
+            order_id=order.id,
+            order_number=order.order_number,
+            reason=reason,
+            created_by="system",
+        )
+        db.add(transaction)
+        return transaction
+
     def reserve_for_order(
         self,
         db: Session,
@@ -142,9 +190,23 @@ class OrderInventoryService:
                     location=location,
                     quantity=item.quantity,
                 )
-                inventory_service.reserve_inventory(db, vendor_id, reserve_data)
+                updated_inventory = inventory_service.reserve_inventory(
+                    db, vendor_id, reserve_data
+                )
                 reserved_count += 1
 
+                # Log transaction for audit trail
+                self._log_transaction(
+                    db=db,
+                    vendor_id=vendor_id,
+                    product_id=item.product_id,
+                    inventory=updated_inventory,
+                    transaction_type=TransactionType.RESERVE,
+                    quantity_change=0,  # Reserve doesn't change quantity, only reserved_quantity
+                    order=order,
+                    reason=f"Reserved for order {order.order_number}",
+                )
+
                 logger.info(
                     f"Reserved {item.quantity} units of product {item.product_id} "
                     f"for order {order.order_number}"
@@ -242,9 +304,23 @@ class OrderInventoryService:
                     location=location,
                     quantity=item.quantity,
                 )
-                inventory_service.fulfill_reservation(db, vendor_id, reserve_data)
+                updated_inventory = inventory_service.fulfill_reservation(
+                    db, vendor_id, reserve_data
+                )
                 fulfilled_count += 1
 
+                # Log transaction for audit trail
+                self._log_transaction(
+                    db=db,
+                    vendor_id=vendor_id,
+                    product_id=item.product_id,
+                    inventory=updated_inventory,
+                    transaction_type=TransactionType.FULFILL,
+                    quantity_change=-item.quantity,  # Negative because stock is consumed
+                    order=order,
+                    reason=f"Fulfilled for order {order.order_number}",
+                )
+
                 logger.info(
                     f"Fulfilled {item.quantity} units of product {item.product_id} "
                     f"for order {order.order_number}"
@@ -335,9 +411,23 @@ class OrderInventoryService:
                     location=inventory.location,
                     quantity=item.quantity,
                 )
-                inventory_service.release_reservation(db, vendor_id, reserve_data)
+                updated_inventory = inventory_service.release_reservation(
+                    db, vendor_id, reserve_data
+                )
                 released_count += 1
 
+                # Log transaction for audit trail
+                self._log_transaction(
+                    db=db,
+                    vendor_id=vendor_id,
+                    product_id=item.product_id,
+                    inventory=updated_inventory,
+                    transaction_type=TransactionType.RELEASE,
+                    quantity_change=0,  # Release doesn't change quantity, only reserved_quantity
+                    order=order,
+                    reason=f"Released for cancelled order {order.order_number}",
+                )
+
                 logger.info(
                     f"Released {item.quantity} units of product {item.product_id} "
                     f"for cancelled order {order.order_number}"