Add a repeatable way to seed test data

This is an internal change with no effect on live workspaces. docs/SEED_DATA.md records the exact table shapes, JSON formats and SQL patterns needed to generate fake contacts, kanban boards, documents, spreadsheets, to-do lists and notes for a single user and workspace, in a form that can be handed to an assistant to produce a fresh batch on demand. migrations/744_seed_demo_workspace_data.sql is the first batch and inserts nothing outside the workspace it names. Seed migrations are optional and can be skipped in any environment that should not carry demo data.