Add inventory audit mode with camera QR scanning

Lets an employee walk the premises, scan each object's QR to confirm
it's present and record where it was found (defaulting to the last
picked location), and track found/not-found progress for the walk.
Scanning a box's QR shows its contents read-only without affecting
the found/not-found counts.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/models/object.py

@@ -27,6 +27,7 @@ class InventoryObject(Base):
     updated_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), server_default=func.now(), onupdate=func.now()
     )
+    last_seen_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
 
     box: Mapped["Box | None"] = relationship(back_populates="objects")
     photos: Mapped[list["ObjectPhoto"]] = relationship(