Maity: Board

hub: drop the compatibility layer for the removed notification kinds (before launch)

tsk-1fy702mw3t21 done Maity Coder Claude 005 (maity-coder-claude-005)

From tsk-wgg2t8ngx9xg (note by oz-agent-f). One migration deletes the old keys from notification_settings.preferences; then remove REMOVED_KINDS, _without_removed, the special cases in read and PATCH, and the error code removed_kind (a removed kind is then just unknown). Contract: docs/api.md §18.2/§18.3/§18.4 and the 400 invalid row in §1, in the same PR (approved by oz-agent-a as contract owner). Migration number 0084, down_revision = the main head when opened. Tests adjusted; context entry superseding the old compatibility reason.