Let an end time mean the card leaves the board

The shopfloor feed kept every ended notification up for a hardcoded 30 minutes,
flagged resolved. A card with an 8:00 end time was still on the board at 8:29,
which reads as an expiry that did not work - and in the carousel, grid and
banner sections it read that way with no visual sign at all, since only the
standard cards render the resolved state.

The tail is now notificationtypes.gracewindowminutes, set per type on the
Notification Types page and defaulting to 0, so an end time means what it says.
A type whose cards are worth acknowledging after they clear - an incident, say
- opts into a tail, and only that type's cards get one.

The feed widens its query to the largest configured tail and then holds each
row to its own type's window. That keeps one portable query rather than a
per-type interval expression in SQL, and with every type at 0 it collapses to
"still showing".

Also fixes resolved serializing as null rather than false for a card with no
end time, which the and-chain produced.

plugins/notifications/models/notification.py

@@ -75,6 +75,14 @@ class NotificationType(db.Model):
     showemployeephoto = db.Column(db.Boolean, default=False)
     displaystyle = db.Column(db.String(20), default='standard')
 
+    # Minutes a card stays on the shopfloor board AFTER its end time, shown as
+    # RESOLVED. 0 (the default) means it leaves the board the moment it ends,
+    # which is what a reader expects from an end time. A type whose cards are
+    # worth acknowledging after the fact - an incident that just cleared - can
+    # opt into a tail.
+    gracewindowminutes = db.Column(db.Integer, nullable=False,
+                                   server_default='0', default=0)
+
     def __repr__(self):
         return f"<NotificationType {self.typename}>"
 
@@ -91,7 +99,8 @@ class NotificationType(db.Model):
             'expiryminute': self.expiryminute if self.expiryminute is not None else 0,
             'splitperemployee': bool(self.splitperemployee),
             'showemployeephoto': bool(self.showemployeephoto),
-            'displaystyle': self.displaystyle or 'standard'
+            'displaystyle': self.displaystyle or 'standard',
+            'gracewindowminutes': int(self.gracewindowminutes or 0)
         }