docs(database): define Citus table groups

[KurodaKayn]

Change Description

• Marks the Phase 5 Citus table-group design item complete and raises the database plan progress to about 80%.
• Adds script/db/citus_table_groups.yml with the tenant_workspace colocated group, reference-table candidates, control-domain exclusions, and deferred colocation gaps.
• Documents the initial Citus validation DDL shape while leaving unique/FK review and worker payload routing as remaining Phase 5 work.

Implementation Approach

• Keeps tenant-owned tables on the workspace UUID value, using workspaces.id for the root table and workspace_id for child tables.
• Keeps platform account, browser session, outbox, and idempotency-claim tables outside the first distributed group until their ownership/routing changes land.
• Adds a Ruby manifest check that validates table classifications, distribution columns, reference DDL markers, and required Phase 5 anchor tables.

Testing

• ruby script/db/test_citus_table_groups.rb (pass)
• Standard product CI lanes are not selected by the current script/ci/select-area.sh patterns for documentation plus script/db changes.