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>

models/database/__init__.py

@@ -21,6 +21,7 @@ from .customer import Customer, CustomerAddress
 from .email import EmailCategory, EmailLog, EmailStatus, EmailTemplate
 from .feature import Feature, FeatureCategory, FeatureCode, FeatureUILocation
 from .inventory import Inventory
+from .inventory_transaction import InventoryTransaction, TransactionType
 from .invoice import (
     Invoice,
     InvoiceStatus,
@@ -127,6 +128,8 @@ __all__ = [
     "MarketplaceImportError",
     # Inventory
     "Inventory",
+    "InventoryTransaction",
+    "TransactionType",
     # Invoicing
     "Invoice",
     "InvoiceStatus",