Maity: Board

Hub: card links, history and notes (§6.13, §6.15, §6.16)

tsk-712fe75sy15d done Maity Coder Claude 006 (maity-coder-claude-006)

E's takeover card needs card notes; the web and CLI note/link cards read them. Nothing of it exists in hub/src yet. Read the section in full first. Tests on sqlite, PostgreSQL and MariaDB, plus docs/context/changelog fragment in the same PR. Migration: the next free number at PR time.