Keep EMA unread status out of public searches

[akirk]

Summary

• register the EMA unread DM status as non-public, matching the read status
• add a regression test that normal WordPress public search SQL does not include ema_unread

Context

Issue #279 shows a normal WordPress front-end search query, not a Mastodon API request. The query included post_status = ema_unread because EMA registered that status as public.

Fixes #279.

Tests

• composer test -- --filter PublicSearch_Test
• composer test -- --filter ConversationsEndpoint_Test
• composer test -- --filter "PublicSearch_Test|ConversationsEndpoint_Test|NotificationsEndpoint_Test"

Note: these focused tests passed before opening the PR. A later accidental shell execution while creating the PR body reran the conversation-related tests and hit an environment-specific OpenSSL random state write error from the ActivityPub plugin.