feat: add billing pricing rules

apps/billing/admin.py

@@ -1,6 +1,6 @@
 from django.contrib import admin
 
-from .models import CallRecord, PointsLedger
+from .models import CallRecord, ExchangeRate, PointsLedger, PricingRule
 
 
 class ReadOnlyLedgerAdmin(admin.ModelAdmin):
@@ -33,6 +33,41 @@ class PointsLedgerAdmin(ReadOnlyLedgerAdmin):
     ordering = ("-created_at", "-id")
 
 
+@admin.register(PricingRule)
+class PricingRuleAdmin(admin.ModelAdmin):
+    list_display = (
+        "operation_type",
+        "alias",
+        "resolution_display",
+        "points_cost",
+        "is_active",
+        "updated_at",
+    )
+    list_filter = ("operation_type", "is_active")
+    search_fields = ("alias", "resolution")
+    ordering = ("operation_type", "alias", "resolution")
+    readonly_fields = ("created_at", "updated_at")
+
+    @admin.display(description="resolution")
+    def resolution_display(self, obj):
+        return obj.resolution or "*"
+
+
+@admin.register(ExchangeRate)
+class ExchangeRateAdmin(admin.ModelAdmin):
+    list_display = (
+        "currency",
+        "points_per_unit",
+        "is_active",
+        "effective_from",
+        "updated_at",
+    )
+    list_filter = ("currency", "is_active")
+    search_fields = ("currency", "note")
+    ordering = ("-effective_from", "-id")
+    readonly_fields = ("created_at", "updated_at")
+
+
 @admin.register(CallRecord)
 class CallRecordAdmin(ReadOnlyLedgerAdmin):
     list_display = (