Add DB-level defaults for columns that deserve them

A prime example is the `:active` column we have for lots of models like Table, which has a `NOT NULL` constraint but no default value. A new table is obviously active when we insert it into the DB so we should make the default value of `:active` be `true`.

Start by locating the Table model and the database schema or migration defining its :active column. Review other NOT NULL columns to determine which defaults belong at the database level, then verify that new inserts receive the intended defaults without weakening the constraints.

Written by the indexing model from the issue text.