state defaults to 'planning' (server default plus SQLAlchemy default).
activated_at and closed_at are nullable timestamps that record when
the month crossed each boundary. Alembic batch_alter_table handles
the SQLite rewrite. MonthState is a Python string enum mapped to a
non-native VARCHAR(16).
Refs #15
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
