Replace box picker with free-text location in audit scan

"Where found" during inventory audit is now a plain text field
(e.g. "каб. 305") stored on the object instead of assigning it to a
box, since audit walks often use room/office labels that don't map
to the box/shelf hierarchy used elsewhere. Box assignment via the
regular object form is unaffected.

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

backend/app/models/object.py

@@ -28,6 +28,7 @@ class InventoryObject(Base):
         DateTime(timezone=True), server_default=func.now(), onupdate=func.now()
     )
     last_seen_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
+    last_seen_location: Mapped[str | None] = mapped_column(String(255), nullable=True)
 
     box: Mapped["Box | None"] = relationship(back_populates="objects")
     photos: Mapped[list["ObjectPhoto"]] = relationship(