feat: add admin API endpoints for inventory transaction history

Adds admin endpoints for viewing transaction history across all vendors:
- GET /admin/inventory/transactions - paginated cross-vendor list
- GET /admin/inventory/transactions/stats - platform-wide statistics

Includes vendor details in transaction items and transaction counts
by type for dashboard displays.

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

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

models/schema/inventory.py

@@ -263,3 +263,32 @@ class OrderTransactionHistoryResponse(BaseModel):
     order_id: int
     order_number: str
     transactions: list[InventoryTransactionWithProduct]
+
+
+# ============================================================================
+# Admin Inventory Transaction Schemas
+# ============================================================================
+
+
+class AdminInventoryTransactionItem(InventoryTransactionWithProduct):
+    """Transaction with vendor details for admin views."""
+
+    vendor_name: str | None = None
+    vendor_code: str | None = None
+
+
+class AdminInventoryTransactionListResponse(BaseModel):
+    """Paginated list of transactions for admin."""
+
+    transactions: list[AdminInventoryTransactionItem]
+    total: int
+    skip: int
+    limit: int
+
+
+class AdminTransactionStatsResponse(BaseModel):
+    """Transaction statistics for admin dashboard."""
+
+    total_transactions: int
+    transactions_today: int
+    by_type: dict[str, int]