bug: keep chat messages out of the notifications bell

[lukepolo]

Chat messages were saved as notification rows, so every DM and lobby line also showed up in the notifications bell. Chat now pushes straight to devices without writing any rows.

• Chat pushes go out directly from the message; bundling, quiet hours, focus and read-cursor checks still apply
• Deleting or editing a message updates a push that is still waiting to go out, and a block drops the blocked sender's held messages
• A burst whose later messages were all deleted or read no longer re-sends the first message
• Migration removes existing chat notification rows, the in-app chat preferences and the index that only served them
• The in-app chat toggles are gone from notification settings