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.

tests/test_plugin_migrations.py

@@ -64,8 +64,8 @@ EXPECTED_HEAD_REVISION['network'] = 'network0002model'
 # printedparts is post-cutover: its 0001 really creates its tables; 0004 adds
 # the per-transaction revision column.
 EXPECTED_HEAD_REVISION['printedparts'] = 'printedparts0004txnrev'
-# notifications indexes businessunitid on top of its anchor.
-EXPECTED_HEAD_REVISION['notifications'] = 'notifications0002buidx'
+# notifications indexes businessunitid, then adds the per-type grace window.
+EXPECTED_HEAD_REVISION['notifications'] = 'notifications0003grace'
 
 # Plugins built after the cutover: their 0001 baseline really creates tables the
 # core chain never owned.