feat: add superseded column to Download model

backend/app/models/download.py

@@ -2,7 +2,7 @@
 
 from datetime import datetime
 from typing import Optional
-from sqlalchemy import String, Integer, Text, ForeignKey, BigInteger
+from sqlalchemy import String, Integer, Text, ForeignKey, BigInteger, Boolean
 from sqlalchemy.dialects.postgresql import JSONB
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 
@@ -55,6 +55,9 @@ class Download(Base, TimestampMixin):
     file_count: Mapped[int] = mapped_column(Integer, default=0)
     total_size: Mapped[Optional[int]] = mapped_column(BigInteger, nullable=True)
 
+    # Superseded: True if a later successful download has run for this source
+    superseded: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False, server_default="false")
+
     # Timing
     started_at: Mapped[Optional[datetime]] = mapped_column(nullable=True)
     completed_at: Mapped[Optional[datetime]] = mapped_column(nullable=True)
@@ -82,6 +85,7 @@ class Download(Base, TimestampMixin):
             "total_size": self.total_size,
             "started_at": format_datetime(self.started_at),
             "completed_at": format_datetime(self.completed_at),
+            "superseded": self.superseded,
             "metadata": self.metadata_,
             "created_at": format_datetime(self.created_at),
         }