* feat: Add seen/unseen activity center state
* feat: ActivityCenterState for grouping ActivityCenter unread messages cnt and seen state
* feat: always use messenger's addActivityCenterNotification & add state to the response
* Remove unused activity center endpoints form api and fix test
We only write up migrations, down migrations are not always possible in sqlite or
too complex/too expensive. For example to remove a column you would have to duplicate
the table, copy over the data, delete and recreated.
This can be very expensive for some tables (user_messages for example), so should not be attempted.
Notes
One issue we faced multiple times is that updates to user_messages can be very
expensive, leading to slow upgrade times and interrupted migrations. So avoid
writes if not necessary.
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