feat(webapp,database): save platform notifications as drafts and publish later (#4743)

## Summary

The platform notifications admin page can now save a notification as a
draft without committing to a schedule, then publish it later by
entering start and end dates. Drafts stay hidden from the webapp panel,
the CLI, and the "What's new" changelog until they are published.

## Design

A draft is an `isDraft` flag on `PlatformNotification`, not nullable
dates, so the existing index and every read query stay intact. All three
reader queries filter on the flag, so a draft can never surface
regardless of its placeholder dates. Publishing writes the real start
and end dates and clears the flag; the publish dialog validates the
range and shows inline errors. Editing a draft keeps it a draft, with
the schedule fields hidden until publish.

Also folds in a small tweak: the "Send preview to me" test button now
appears when editing a notification, not just when creating one.

Author: D-K-P