Amends §7.2 inline and marks phase 8 done in Part 12; adds the
team_integration_config row to BACKEND_DESIGN.md's schema table.
Two of the amendments are things the tree disproved rather than choices:
- §7.2's DDL cannot hold its own default row. MariaDB coerces PRIMARY KEY
columns to NOT NULL, so `team_id NULL` is unrepresentable and the override
mechanism has no base case. Confirmed against a real MariaDB (error 1048).
- §7.2's visibility gate has no data source on either side and cannot have one:
the streams carry no visibility, a forum thread is members-only by
construction rather than by a column, and core cannot see a channel's
permissions. The gate becomes an attributed operator acknowledgement.
Co-Authored-By: Claude <noreply@anthropic.com>