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

plugins/machines/models/machine.py

@@ -73,8 +73,12 @@ class Machine(BaseModel):
     )
 
     # Maintenance tracking
-    lastmaintenancedate = db.Column(db.DateTime, nullable=True)
-    nextmaintenancedate = db.Column(db.DateTime, nullable=True)
+    # Dates, not datetimes: there is no time in a maintenance date, and a
+    # DATETIME went out of to_dict as "...T00:00:00Z", which a browser west of
+    # UTC rendered as the previous day. measuringtools and warranties already
+    # model theirs as db.Date. See migration 7d40.
+    lastmaintenancedate = db.Column(db.Date, nullable=True)
+    nextmaintenancedate = db.Column(db.Date, nullable=True)
     maintenanceintervaldays = db.Column(db.Integer, nullable=True)
 
     # Controller info (for CNC machines)