feat: billing for groups of deals

models/deal_group.py

@@ -4,6 +4,7 @@ from sqlalchemy import ForeignKey, Table, Column
 from sqlalchemy.orm import mapped_column, Mapped, relationship
 
 from models import BaseModel
+from models import GroupBillRequest
 
 if TYPE_CHECKING:
     from models import Deal
@@ -24,6 +25,10 @@ class DealGroup(BaseModel):
         back_populates='group',
         secondary='deal_relations'
     )
+    bill_request: Mapped[Optional['GroupBillRequest']] = relationship(
+        back_populates='group',
+        lazy='joined'
+    )
 
 
 deal_relations = Table(