DB review fixes: drop redundant indexes + dead column, add CI MySQL-upgrade job

From the database review (verdict: sound-with-minor-issues). Applies the
actionable findings.

Redundant indexes: five non-unique secondary indexes duplicated a named idx_*
or a unique index on the same column - ix_communications_assetid,
ix_computers_hostname, ix_networkdevices_hostname, ix_printers_hostname (each
shadowing an idx_*), and idx_usb_serial (shadowing the serialnumber unique
index). Removed the redundant index source from the models (column index=True /
the extra db.Index) and added core migration 7d25 dropping the live duplicates.
The unique ix_*_assetid indexes are kept (they enforce assetid uniqueness).

Dead column: usbcheckouts.machineid was a NOT NULL soft-ref to the retired
machines table storing sentinel 0 (ADR-001). Dropped from the model + the
machineid=0 literal in selfhosted checkout; usb plugin migration 0002 drops it
live (downgrade restores it default 0).

Index: notifications.businessunitid (filtered by the shopfloor feed) was
unindexed; added index=True + notifications migration 0002.

CI: new migrations-mysql job proves the real multi-site deploy path - fresh
`flask db upgrade` + per-plugin install on utf8mb4 MySQL from empty, asserting
table count + charset and a clean second-run no-op. The pytest suite only
exercises SQLite create_all(), so a regression in the Alembic chain on MySQL
would otherwise ship undetected.

Verified: fresh core upgrade on a scratch utf8mb4 MySQL builds clean + no-op on
rerun (redundant indexes absent, unique assetid kept); plugin migrations applied
+ verified on the dev DB (machineid gone, bu index present). 953 backend tests
pass; naming + pyflakes green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

plugins/usb/migrations/versions/0002_drop_checkout_machineid.py

@@ -0,0 +1,42 @@
+"""Drop the dead usbcheckouts.machineid column (ADR-001: Machine retired).
+
+machineid was a NOT NULL soft-reference to the retired machines table; every
+new checkout stored the sentinel 0. It has no FK, no meaning under the asset
+model, and no reader. Drop it.
+
+Idempotent (skips a missing table/column). Downgrade restores it NOT NULL with
+server_default 0 so any existing rows stay valid.
+
+Revision ID: usb0002dropmachineid
+Revises: usb0001anchor
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+revision = 'usb0002dropmachineid'
+down_revision = 'usb0001anchor'
+branch_labels = None
+depends_on = None
+
+
+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')}
+    if 'machineid' in cols:
+        op.drop_column('usbcheckouts', 'machineid')
+
+
+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')}
+    if 'machineid' not in cols:
+        op.add_column('usbcheckouts',
+                      sa.Column('machineid', sa.Integer(), nullable=False,
+                                server_default='0'))