feat: add AirShelf core implementation

core/backend/apps/billing/serializers.py

@@ -0,0 +1,43 @@
+from rest_framework import serializers
+
+from .models import CreditAccount, CreditLedger, CreditReservation, QuotaPolicy
+
+
+class CreditAccountSerializer(serializers.ModelSerializer):
+    class Meta:
+        model = CreditAccount
+        fields = ["id", "balance", "reserved_balance", "currency", "updated_at"]
+        read_only_fields = fields
+
+
+class CreditLedgerSerializer(serializers.ModelSerializer):
+    class Meta:
+        model = CreditLedger
+        fields = [
+            "id",
+            "user",
+            "project",
+            "task",
+            "ledger_type",
+            "amount",
+            "balance_after",
+            "reason",
+            "metadata",
+            "created_at",
+        ]
+        read_only_fields = fields
+
+
+class CreditReservationSerializer(serializers.ModelSerializer):
+    class Meta:
+        model = CreditReservation
+        fields = ["id", "user", "project", "task", "amount", "status", "expires_at", "created_at"]
+        read_only_fields = fields
+
+
+class QuotaPolicySerializer(serializers.ModelSerializer):
+    class Meta:
+        model = QuotaPolicy
+        fields = ["id", "user", "project", "monthly_limit", "project_limit", "per_task_limit", "is_active"]
+        read_only_fields = ["id"]
+