fix: avoid MySQL timezone dependency in billing admin

apps/billing/admin.py

@@ -31,7 +31,6 @@ class PointsLedgerAdmin(ReadOnlyLedgerAdmin):
     list_filter = ("change_type", "created_at")
     search_fields = ("user__username", "user__email", "=ref_order_id", "reason")
     ordering = ("-created_at", "-id")
-    date_hierarchy = "created_at"
     list_select_related = ("user", "ref_call")
 
 
@@ -70,7 +69,6 @@ class ExchangeRateAdmin(admin.ModelAdmin):
     ordering = ("-effective_from", "-id")
     readonly_fields = ("created_at", "updated_at")
     list_editable = ("is_active",)
-    date_hierarchy = "effective_from"
 
 
 @admin.register(RechargeOrder)
@@ -95,7 +93,6 @@ class RechargeOrderAdmin(ReadOnlyLedgerAdmin):
         "payment_txn_no",
     )
     ordering = ("-created_at", "-id")
-    date_hierarchy = "created_at"
     list_select_related = ("user",)
 
 
@@ -125,5 +122,4 @@ class CallRecordAdmin(ReadOnlyLedgerAdmin):
         "result_summary",
     )
     ordering = ("-created_at", "-id")
-    date_hierarchy = "created_at"
     list_select_related = ("user", "api_key")