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>

docs/implementation/stock-management-integration.md

@@ -188,9 +188,63 @@ INFO: Fulfilled 2 units of product 123 for order ORD-1-20260101-ABC123
 WARNING: Order ORD-1-20260101-ABC123 inventory operation failed: No inventory found
 ```
 
+## Audit Trail (Phase 2)
+
+All inventory operations are logged to the `inventory_transactions` table.
+
+### Transaction Types
+
+| Type | Description |
+|------|-------------|
+| `reserve` | Stock reserved for order |
+| `fulfill` | Reserved stock consumed (shipped) |
+| `release` | Reserved stock released (cancelled) |
+| `adjust` | Manual adjustment (+/-) |
+| `set` | Set to exact quantity |
+| `import` | Initial import/sync |
+| `return` | Stock returned from customer |
+
+### Transaction Record
+
+```python
+class InventoryTransaction:
+    id: int
+    vendor_id: int
+    product_id: int
+    inventory_id: int | None
+    transaction_type: TransactionType
+    quantity_change: int       # Positive = add, negative = remove
+    quantity_after: int        # Snapshot after transaction
+    reserved_after: int        # Snapshot after transaction
+    location: str | None
+    warehouse: str | None
+    order_id: int | None       # Link to order if applicable
+    order_number: str | None
+    reason: str | None         # Human-readable reason
+    created_by: str | None     # User/system identifier
+    created_at: datetime
+```
+
+### Example Transaction Query
+
+```python
+from models.database import InventoryTransaction, TransactionType
+
+# Get all transactions for an order
+transactions = db.query(InventoryTransaction).filter(
+    InventoryTransaction.order_id == order_id
+).order_by(InventoryTransaction.created_at).all()
+
+# Get recent stock changes for a product
+recent = db.query(InventoryTransaction).filter(
+    InventoryTransaction.product_id == product_id,
+    InventoryTransaction.vendor_id == vendor_id,
+).order_by(InventoryTransaction.created_at.desc()).limit(10).all()
+```
+
 ## Future Enhancements
 
-1. **Inventory Transaction Log** - Audit trail for all stock movements
-2. **Multi-Location Selection** - Choose which location to draw from
-3. **Backorder Support** - Handle orders when stock is insufficient
-4. **Return Processing** - Increase stock when orders are returned
+1. **Multi-Location Selection** - Choose which location to draw from
+2. **Backorder Support** - Handle orders when stock is insufficient
+3. **Return Processing** - Increase stock when orders are returned
+4. **Transaction API** - Endpoint to view inventory history