US06-03: Preserve Offline Identity and Review Evidence

photo_pipeline/models/thumbnails.py

@@ -11,7 +11,7 @@ from __future__ import annotations
 
 from datetime import datetime
 
-from sqlalchemy import DateTime, ForeignKey, Integer, String, func
+from sqlalchemy import Boolean, DateTime, ForeignKey, Integer, String, func
 from sqlalchemy.orm import Mapped, mapped_column
 
 from photo_pipeline.db import Base
@@ -29,6 +29,9 @@ class Thumbnail(Base):
     width: Mapped[int | None] = mapped_column(Integer)
     height: Mapped[int | None] = mapped_column(Integer)
     format: Mapped[str | None] = mapped_column(String)
+    # Durable comparison evidence for an archived asset: never evicted by the LRU
+    # quota, because the original may be on a medium that is no longer reachable.
+    protected: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False)
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), nullable=False, server_default=func.now()
     )