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>

app/services/inventory_transaction_service.py

@@ -289,5 +289,143 @@ class InventoryTransactionService:
         }
 
 
+    # =========================================================================
+    # Admin Methods (cross-vendor operations)
+    # =========================================================================
+
+    def get_all_transactions_admin(
+        self,
+        db: Session,
+        skip: int = 0,
+        limit: int = 50,
+        vendor_id: int | None = None,
+        product_id: int | None = None,
+        transaction_type: str | None = None,
+        order_id: int | None = None,
+    ) -> tuple[list[dict], int]:
+        """
+        Get inventory transactions across all vendors (admin only).
+
+        Args:
+            db: Database session
+            skip: Pagination offset
+            limit: Pagination limit
+            vendor_id: Optional vendor filter
+            product_id: Optional product filter
+            transaction_type: Optional transaction type filter
+            order_id: Optional order filter
+
+        Returns:
+            Tuple of (transactions with details, total count)
+        """
+        from models.database.vendor import Vendor
+
+        # Build query
+        query = db.query(InventoryTransaction)
+
+        # Apply filters
+        if vendor_id:
+            query = query.filter(InventoryTransaction.vendor_id == vendor_id)
+        if product_id:
+            query = query.filter(InventoryTransaction.product_id == product_id)
+        if transaction_type:
+            query = query.filter(
+                InventoryTransaction.transaction_type == transaction_type
+            )
+        if order_id:
+            query = query.filter(InventoryTransaction.order_id == order_id)
+
+        # Get total count
+        total = query.count()
+
+        # Get transactions with pagination (newest first)
+        transactions = (
+            query.order_by(InventoryTransaction.created_at.desc())
+            .offset(skip)
+            .limit(limit)
+            .all()
+        )
+
+        # Build result with vendor and product details
+        result = []
+        for tx in transactions:
+            vendor = db.query(Vendor).filter(Vendor.id == tx.vendor_id).first()
+            product = db.query(Product).filter(Product.id == tx.product_id).first()
+
+            product_title = None
+            product_sku = None
+            if product:
+                product_sku = product.vendor_sku
+                if product.marketplace_product:
+                    product_title = product.marketplace_product.get_title()
+
+            result.append(
+                {
+                    "id": tx.id,
+                    "vendor_id": tx.vendor_id,
+                    "vendor_name": vendor.name if vendor else None,
+                    "vendor_code": vendor.vendor_code if vendor else None,
+                    "product_id": tx.product_id,
+                    "inventory_id": tx.inventory_id,
+                    "transaction_type": (
+                        tx.transaction_type.value if tx.transaction_type else None
+                    ),
+                    "quantity_change": tx.quantity_change,
+                    "quantity_after": tx.quantity_after,
+                    "reserved_after": tx.reserved_after,
+                    "location": tx.location,
+                    "warehouse": tx.warehouse,
+                    "order_id": tx.order_id,
+                    "order_number": tx.order_number,
+                    "reason": tx.reason,
+                    "created_by": tx.created_by,
+                    "created_at": tx.created_at,
+                    "product_title": product_title,
+                    "product_sku": product_sku,
+                }
+            )
+
+        return result, total
+
+    def get_transaction_stats_admin(self, db: Session) -> dict:
+        """
+        Get transaction statistics across the platform (admin only).
+
+        Returns:
+            Dict with transaction counts by type
+        """
+        from sqlalchemy import func as sql_func
+
+        # Count by transaction type
+        type_counts = (
+            db.query(
+                InventoryTransaction.transaction_type,
+                sql_func.count(InventoryTransaction.id).label("count"),
+            )
+            .group_by(InventoryTransaction.transaction_type)
+            .all()
+        )
+
+        # Total transactions
+        total = db.query(sql_func.count(InventoryTransaction.id)).scalar() or 0
+
+        # Transactions today
+        from datetime import UTC, datetime, timedelta
+
+        today_start = datetime.now(UTC).replace(hour=0, minute=0, second=0, microsecond=0)
+        today_count = (
+            db.query(sql_func.count(InventoryTransaction.id))
+            .filter(InventoryTransaction.created_at >= today_start)
+            .scalar()
+            or 0
+        )
+
+        return {
+            "total_transactions": total,
+            "transactions_today": today_count,
+            "by_type": {tc.transaction_type.value: tc.count for tc in type_counts},
+        }
+
+
 # Create service instance
 inventory_transaction_service = InventoryTransactionService()