Disambiguate ORDER BY in balance history DISTINCT ON

The SELECT aliased bucket_timestamp AS timestamp, so the secondary
ORDER BY timestamp DESC resolved to the alias (a constant within each
bucket) instead of the source row's timestamp. DISTINCT ON then picked
an arbitrary hour per day. Rename the underlying column to
source_timestamp in the CTE so the ORDER BY references it unambiguously
and end-of-day is selected.

Author: claude

api/v1_users_balance_history.go

@@ -118,15 +118,15 @@ func (app *ApiServer) v1UsersBalanceHistory(c *fiber.Ctx) error {
 					WHEN @granularity::text = 'daily' THEN date_trunc('day', timestamp)
 					ELSE timestamp
 				END AS bucket_timestamp,
-				timestamp,
+				timestamp AS source_timestamp,
 				balance_usd
 			FROM hourly_totals
 		)
 		SELECT DISTINCT ON (bucket_timestamp)
 			bucket_timestamp AS timestamp,
 			balance_usd
 		FROM bucketed
-		ORDER BY bucket_timestamp ASC, timestamp DESC
+		ORDER BY bucket_timestamp ASC, source_timestamp DESC
 	`
 
 	rows, err := app.pool.Query(c.Context(), sql, pgx.NamedArgs{