feat: add vendor API endpoints for inventory transaction history

Adds three new endpoints for viewing stock movement history:
- GET /inventory/transactions - paginated list with filters
- GET /inventory/transactions/product/{id} - product-specific history
- GET /inventory/transactions/order/{id} - order-specific history

Creates InventoryTransactionService to encapsulate query logic
following architecture patterns. Includes response schemas with
product details for rich UI display.

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

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

models/schema/inventory.py

@@ -200,3 +200,66 @@ class AdminInventoryLocationsResponse(BaseModel):
     """Response for unique inventory locations."""
 
     locations: list[str]
+
+
+# ============================================================================
+# Inventory Transaction Schemas
+# ============================================================================
+
+
+class InventoryTransactionResponse(BaseModel):
+    """Single inventory transaction record."""
+
+    model_config = ConfigDict(from_attributes=True)
+
+    id: int
+    vendor_id: int
+    product_id: int
+    inventory_id: int | None = None
+    transaction_type: str
+    quantity_change: int
+    quantity_after: int
+    reserved_after: int
+    location: str | None = None
+    warehouse: str | None = None
+    order_id: int | None = None
+    order_number: str | None = None
+    reason: str | None = None
+    created_by: str | None = None
+    created_at: datetime
+
+
+class InventoryTransactionWithProduct(InventoryTransactionResponse):
+    """Transaction with product details for list views."""
+
+    product_title: str | None = None
+    product_sku: str | None = None
+
+
+class InventoryTransactionListResponse(BaseModel):
+    """Paginated list of inventory transactions."""
+
+    transactions: list[InventoryTransactionWithProduct]
+    total: int
+    skip: int
+    limit: int
+
+
+class ProductTransactionHistoryResponse(BaseModel):
+    """Transaction history for a specific product."""
+
+    product_id: int
+    product_title: str | None = None
+    product_sku: str | None = None
+    current_quantity: int
+    current_reserved: int
+    transactions: list[InventoryTransactionResponse]
+    total: int
+
+
+class OrderTransactionHistoryResponse(BaseModel):
+    """Transaction history for a specific order."""
+
+    order_id: int
+    order_number: str
+    transactions: list[InventoryTransactionWithProduct]