Let the row refuse the write both kiosks were deciding in Python

Wave 1 of the remediation plan for the 2026-09-03 feature test run. Two kiosks,
one shape of defect, one critical consequence.

Both kiosks read the state, decided in Python, and then wrote. Nothing stops two
requests interleaving between the read and the write, so both see the same
pre-state and both proceed. Reproduced on MySQL 5.6 over HTTP, five concurrent
takes of 5 from a bin holding 5: all five were accepted and quantityonhand
landed on -20, because each request wrote its own arithmetic. The ledger had
recorded every one of them, so the cached quantity and the ledger disagreed
permanently, which is the number the shop floor actually reads. The USB kiosk
had the same shape: concurrent scans of one drive all succeeded and inserted
several open custody rows, so one drive was recorded out to two people and the
documented 409 never fired.

The comparison now lives in the WHERE clause, where the database settles it at
write time. printedparts._move_quantity issues
UPDATE ... WHERE quantityonhand >= :n and treats rowcount 0 as the refusal,
raising InsufficientStock, which take and adjust turn back into the message they
already gave - carrying the quantity the database holds now, not the stale one
the request read. The low-stock alert derives its before-value from what the
write produced for the same reason. Both USB paths claim the device with
ischeckedout (self-hosted) or status (external cmmc_usb) in the WHERE clause;
the external one claims BEFORE writing the log line, so a refused claim leaves
nothing to undo. Check-in is guarded the same way.

After the fix, the same five concurrent takes: one accepted, four refused,
quantityonhand 0 and the ledger sum 0.

Check-in also closes EVERY open row for the device rather than the newest.
One device can only be out once, so a second open row is corruption, and
closing only the newest left the older ones open forever - the device then read
as returned while its custody history said otherwise.

Separately, usbcheckouts now records who handed a drive back
(checkinsso/checkinname, migration usb0004checkinidentity). Returning for a
colleague is a supported flow that the kiosk asks to confirm, but the row it
wrote attributed the check-in to the HOLDER, so the custody history and the
audit report both named somebody who was not there. Nullable and deliberately
not backfilled: copying the holder's badge into old rows would manufacture
evidence for the exact claim this stops making.

Four tests. Both guards verified by planting: strip the condition out of either
UPDATE and its test fails.

Scope: product

plugins/usb/migrations/versions/0004_checkin_identity.py

@@ -0,0 +1,59 @@
+"""Record who handed a drive back, not just when.
+
+usbcheckouts held the checkout identity (sso, checkoutname) and, for the
+return, only checkintime. Returning a drive for a colleague is a supported
+kiosk flow - usb_kiosk_allow_return_for_others gates it and the kiosk asks for
+confirmation - but the row it wrote attributed the check-in to the HOLDER. The
+person who actually walked the drive back was recorded nowhere, so the custody
+history and the audit report both named somebody who was not there.
+
+Two columns rather than one: the badge is what the scanner emits and what joins
+to a directory, and the name is what a report has to print when the directory
+no longer resolves that badge.
+
+Nullable, and not backfilled. A row closed before this column existed is one
+where the returner was not recorded; copying the holder's badge into it would
+manufacture evidence that the holder returned it, which is the exact claim this
+migration exists to stop making.
+
+Idempotent, so a re-run on an already-migrated database is a no-op.
+
+Revision ID: usb0004checkinidentity
+Revises: usb0003scanfields
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+revision = 'usb0004checkinidentity'
+down_revision = 'usb0003scanfields'
+branch_labels = None
+depends_on = None
+
+NEW_COLUMNS = (
+    ('checkinsso', sa.String(length=20)),
+    ('checkinname', sa.String(length=100)),
+)
+
+
+def upgrade():
+    bind = op.get_bind()
+    insp = sa.inspect(bind)
+    if 'usbcheckouts' not in insp.get_table_names():
+        return
+    cols = {c['name'] for c in insp.get_columns('usbcheckouts')}
+    for name, coltype in NEW_COLUMNS:
+        if name not in cols:
+            op.add_column('usbcheckouts',
+                          sa.Column(name, coltype, nullable=True))
+
+
+def downgrade():
+    bind = op.get_bind()
+    insp = sa.inspect(bind)
+    if 'usbcheckouts' not in insp.get_table_names():
+        return
+    cols = {c['name'] for c in insp.get_columns('usbcheckouts')}
+    for name, _ in NEW_COLUMNS:
+        if name in cols:
+            op.drop_column('usbcheckouts', name)