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>

migrations/versions/7d25_drop_redundant_indexes.py

@@ -0,0 +1,57 @@
+"""Drop redundant secondary indexes duplicating a named idx_* / unique index.
+
+The DB review found five non-unique secondary indexes that duplicate an existing
+index on the same column: the auto-named ix_* from a column index=True alongside
+a named idx_* in __table_args__ (communications.assetid, and the hostname on
+computers/networkdevices/printers), plus idx_usb_serial duplicating the
+serialnumber unique index on usbdevices. Each redundant pair costs insert/update
+maintenance with zero read benefit. The models had the redundant index source
+removed; this drops the live duplicates so the schema matches the models.
+
+Idempotent (skips a missing table/index); real downgrade recreates them.
+
+Revision ID: 7d25_drop_redundant_indexes
+Revises: 7d24_customfield_searchable
+Create Date: 2026-07-13
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+revision = '7d25_drop_redundant_indexes'
+down_revision = '7d24_customfield_searchable'
+branch_labels = None
+depends_on = None
+
+# (table, index name, columns, unique) - columns/unique used only to recreate on
+# downgrade. All non-unique; the surviving idx_* / unique index still covers each.
+_REDUNDANT = [
+    ('communications', 'ix_communications_assetid', ['assetid'], False),
+    ('computers', 'ix_computers_hostname', ['hostname'], False),
+    ('networkdevices', 'ix_networkdevices_hostname', ['hostname'], False),
+    ('printers', 'ix_printers_hostname', ['hostname'], False),
+    ('usbdevices', 'idx_usb_serial', ['serialnumber'], False),
+]
+
+
+def _index_names(insp, table):
+    return {i['name'] for i in insp.get_indexes(table)}
+
+
+def upgrade():
+    bind = op.get_bind()
+    insp = sa.inspect(bind)
+    tables = set(insp.get_table_names())
+    for table, name, _cols, _unique in _REDUNDANT:
+        if table in tables and name in _index_names(insp, table):
+            op.drop_index(name, table_name=table)
+
+
+def downgrade():
+    bind = op.get_bind()
+    insp = sa.inspect(bind)
+    tables = set(insp.get_table_names())
+    for table, name, cols, unique in _REDUNDANT:
+        if table in tables and name not in _index_names(insp, table):
+            op.create_index(name, table, cols, unique=unique)