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/api/selfhosted.py

@@ -198,7 +198,7 @@ def checkout_device(device_id, data):
         override = parse_import_datetime(data.get('checkouttime'))
         if override is not None:
             eventtime = override
-    db.session.add(USBCheckout(usbdeviceid=device.usbdeviceid, machineid=0, sso=badge,
+    db.session.add(USBCheckout(usbdeviceid=device.usbdeviceid, sso=badge,
                                checkoutname=name, checkouttime=eventtime,
                                checkoutreason=data.get('reason')))
     device.ischeckedout = True