feat: add monthly export statistics tracking with revenue breakdown

Implement a new statistics module that records export operations per month,
including total revenue, item count, and average order value. The module
aggregates data from the orders and shipments tables, grouping results by
calendar month for reporting purposes.

src/ragnar/api.py

@@ -70,7 +70,7 @@ class Api:
                 "token_key": self.auth["token_key"],
                 "vote": vote,
                 "plat": 3,
-                "reason": 2
+                "reason": 2,
             },
         )
         return json.loads(response.text)