Store and serve a maintenance date as a date

The machine maintenance panel showed every date one calendar day early on this
site, and would show it a day late east of UTC.

lastmaintenancedate and nextmaintenancedate were DATETIME columns holding
midnight, and BaseModel.to_dict serialises a datetime as isoformat + 'Z', so
2026-12-15 00:00:00 went out as "2026-12-15T00:00:00Z" and the browser
converted it into local time. There is no time in a maintenance date: the form
offers a date picker, the CSV importer takes '2026-06-01', and the value is only
ever compared against other dates. Every sibling already models it correctly -
measuringtools.lastcalibrationdate, warranties.startdate and enddate are all
db.Date - so machines were the odd one out.

Migration 7d40 converts both columns; MySQL truncates a midnight time so no
data moves, and SQLite neither enforces nor needs it.

That exposed two more things. The route was assigning the raw request string to
the column, which a DATE column rejects outright - it now goes through the
strict parser from wave 2, so a mistyped maintenance date is refused with a 400
rather than stored or nulled. And BaseModel.to_dict had no branch for a plain
date, so one reached Flask's encoder and came out as an HTTP-date
("Thu, 15 Jan 2026 00:00:00 GMT") - which a browser parses back into a moment
and can still render as the day before. Dates now serialise as "2026-01-15",
with datetime checked first because it is a subclass of date.

Verified against MySQL on a live instance: the column is DATE, the API serves
2026-01-15, a mistyped date is refused with 400, and a rename on a machine that
has maintenance dates saves - the 500 from the audit-log fix in 5d81f50c, now
confirmed end to end rather than only in tests.

Scope: product

migrations/versions/7d40_machine_maintenance_dates_are_dates.py

@@ -0,0 +1,73 @@
+"""Store a maintenance date as a DATE, because that is what it is.
+
+lastmaintenancedate and nextmaintenancedate were DATETIME columns holding
+midnight. BaseModel.to_dict serialises a datetime as isoformat + 'Z', so
+2026-12-15 00:00:00 went out as "2026-12-15T00:00:00Z" and a browser west of
+UTC rendered it as 14 December. The maintenance panel showed every date one
+calendar day early on this site, and would show it a day LATE east of UTC.
+
+There is no time in a maintenance date. Nobody records that a machine was
+serviced at midnight; the form offers a date picker, the CSV importer takes
+'2026-06-01', and the value is compared against other dates. Every sibling
+already models it correctly - measuringtools.lastcalibrationdate,
+warranties.startdate and enddate are all db.Date - so this is machines being
+the odd one out rather than a new decision.
+
+As a Date, to_dict emits "2026-12-15" with no timezone marker at all, and
+nothing can shift it.
+
+MySQL truncates the (midnight) time component on the ALTER, so no data moves.
+SQLite stores what it is given and does not enforce column types, so the
+conversion is a no-op there and is skipped.
+
+Revision ID: 7d40_machine_maintenance_dates
+Revises: 7d39_audit_retention_key_merge
+"""
+from alembic import op
+
+import sqlalchemy as sa
+
+
+revision = '7d40_machine_maintenance_dates'
+down_revision = '7d39_audit_retention_key_merge'
+branch_labels = None
+depends_on = None
+
+COLUMNS = ('lastmaintenancedate', 'nextmaintenancedate')
+
+
+def _machines_table(bind):
+    inspector = sa.inspect(bind)
+    if 'machines' not in inspector.get_table_names():
+        return None
+    return {column['name'] for column in inspector.get_columns('machines')}
+
+
+def upgrade():
+    bind = op.get_bind()
+    if bind.dialect.name == 'sqlite':
+        return
+    present = _machines_table(bind)
+    if not present:
+        return
+    for column in COLUMNS:
+        if column in present:
+            op.alter_column('machines', column,
+                            existing_type=sa.DateTime(),
+                            type_=sa.Date(),
+                            existing_nullable=True)
+
+
+def downgrade():
+    bind = op.get_bind()
+    if bind.dialect.name == 'sqlite':
+        return
+    present = _machines_table(bind)
+    if not present:
+        return
+    for column in COLUMNS:
+        if column in present:
+            op.alter_column('machines', column,
+                            existing_type=sa.Date(),
+                            type_=sa.DateTime(),
+                            existing_nullable=True)