Refactor user_retention_explorer_api.py for improved type safety and clarity

- Updated the type hint for the `partition_func` parameter in the `sql_users_volume` function to specify it as a Callable that takes a Set of Tuples and returns a string, enhancing type safety.
- Changed the DataFrame creation for `traders_df` to use a dictionary format for better readability and consistency in data structure.

These changes aim to improve code maintainability and clarity in the user retention explorer API.

Author: goldhaxx

backend/api/user_retention_explorer_api.py

@@ -2,7 +2,7 @@
 
 import os
 from datetime import datetime, timedelta, timezone
-from typing import Dict, List, Tuple, Set, Optional, Any
+from typing import Callable, Dict, List, Tuple, Set, Optional, Any
 
 import pandas as pd
 from dateutil import tz, parser
@@ -142,7 +142,7 @@ def sql_users_volume(
     users: List[str],
     start_dt: datetime,
     end_dt: datetime,
-    partition_func: callable,
+    partition_func: Callable[[Set[Tuple[str, str, str]]], str],
     market_index: Optional[int] = None,
     exclude_market_index: Optional[int] = None
 ) -> str:
@@ -223,7 +223,7 @@ async def calculate_retention_for_market(market_name: str, start_date_str: str)
             }
 
         # --- Volume Calculation ---
-        traders_df = pd.DataFrame(mkt_traders, columns=['user'])
+        traders_df = pd.DataFrame({"user": mkt_traders})
 
         # Define time windows
         initial_window_end = start_date + timedelta(days=NEW_TRADER_WINDOW_DAYS)