Add dispute system

backend/app/models/assignment.py

@@ -10,6 +10,7 @@ class AssignmentStatus(str, Enum):
     ACTIVE = "active"
     COMPLETED = "completed"
     DROPPED = "dropped"
+    RETURNED = "returned"  # Disputed and needs to be redone
 
 
 class Assignment(Base):
@@ -34,3 +35,4 @@ class Assignment(Base):
     participant: Mapped["Participant"] = relationship("Participant", back_populates="assignments")
     challenge: Mapped["Challenge"] = relationship("Challenge", back_populates="assignments")
     event: Mapped["Event | None"] = relationship("Event", back_populates="assignments")
+    dispute: Mapped["Dispute | None"] = relationship("Dispute", back_populates="assignment", uselist=False)