printedparts: QR label with gage tag + revision; record taken rev

Label switches from CODE128 to a QR encoding 'TAG|rev' (gage lab tag + latest
print-file revision), so a physical part carries which revision it was printed
from - short payload stays low-version + reliable at 0.5in (margin quiet zone,
EC M, no logo). Item exposes latestrevision; kiosk strips the |rev to resolve
and records the scanned revision on the take (migration 0004 adds
printeditemtransactions.revision) for traceability of which rev was consumed.
Manual entry records a null revision.

plugins/printedparts/migrations/versions/0004_transaction_revision.py

@@ -0,0 +1,43 @@
+"""printedparts: printeditemtransactions.revision (traceability)
+
+Records which print-file revision the physical part carried when it was taken,
+captured from the label QR (gage tag + revision). Nullable - manual entry and
+pre-QR labels have no revision. Idempotent guard.
+
+Revision ID: printedparts0004txnrev
+Revises: printedparts0003gagetag
+Create Date: 2026-07-21
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+revision = 'printedparts0004txnrev'
+down_revision = 'printedparts0003gagetag'
+branch_labels = None
+depends_on = None
+
+
+def _has_col(insp):
+    return any(c['name'] == 'revision'
+               for c in insp.get_columns('printeditemtransactions'))
+
+
+def upgrade():
+    bind = op.get_bind()
+    insp = sa.inspect(bind)
+    if 'printeditemtransactions' not in insp.get_table_names():
+        return
+    if not _has_col(insp):
+        op.add_column('printeditemtransactions',
+                      sa.Column('revision', sa.Integer(), nullable=True))
+
+
+def downgrade():
+    bind = op.get_bind()
+    insp = sa.inspect(bind)
+    if 'printeditemtransactions' not in insp.get_table_names():
+        return
+    if _has_col(insp):
+        op.drop_column('printeditemtransactions', 'revision')