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/alembic/versions/c3bc00a214e0_add_last_seen_at_to_objects.py

@@ -0,0 +1,30 @@
+"""add last_seen_at to objects
+
+Revision ID: c3bc00a214e0
+Revises: cdcadb640107
+Create Date: 2026-07-22 08:05:44.372353
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'c3bc00a214e0'
+down_revision: Union[str, None] = 'cdcadb640107'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.add_column('objects', sa.Column('last_seen_at', sa.DateTime(timezone=True), nullable=True))
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_column('objects', 'last_seen_at')
+    # ### end Alembic commands ###