Rewrite README (single app.db + one alembic_app chain, legacy data moved
once via scripts.migrate_legacy_data, accurate test list) and remove the
Grafana Provisioning section. Update architecture-overview to the unified
data layer (one Base, app-DB engine with WAL) and retire the
alembic_location / alembic_poo sections. Mark M1 done in the roadmap.
Re-export openapi/, which catches the spec up to the already-existing
/config/smtp/test and /public-ip/check endpoints (purely additive; M1's
DB-session dependency swap produced no schema change).
pytest 95 passed; ruff clean (pre-existing only); OpenAPI export idempotent.
tliu93