fix(database): Prevent duplicate primary key on daily stats day rollover

[seer-by-sentry]

This PR addresses the Microsoft.EntityFrameworkCore.DbUpdateException with the inner exception MySqlConnector.MySqlException: Duplicate entry 'PRIMARY' occurring in DailyStatsManager.SaveToDB.

The root cause was identified as the g_StatsContainer static object retaining the previous day's DayOfYear value. When a new calendar day began, SaveToDB would correctly identify that no record existed for the current day, but then attempt to insert g_StatsContainer which still held the previous day's DayOfYear. This led to a primary key violation because a record for the previous day already existed.

The fix involves re-initializing g_StatsContainer with new DailyStat() within the SaveToDB method's entity == null (insert) branch. This ensures that when a new daily stats record is created, g_StatsContainer.DayOfYear is correctly set to the current day, preventing the duplicate key error.

Additionally, the TODO_EFCORE comment regarding preserving old ON DUPLICATE behavior has been removed, as this fix correctly handles the day rollover without needing specific upsert semantics.

Fixes SERVICES-3M