Database opened its sqlite3 connection eagerly in __init__ and closed it
in close(); the lifespan called close() on shutdown. In production this
is fine — the lifespan runs once per process. Under pytest the module-
level ``db`` singleton survives across every TestClient session, so the
second test file's lifespan startup hit
``sqlite3.ProgrammingError: Cannot operate on a closed database`` at
fixture-setup time (AutoBackupEngine.__init__ → db.get_setting("…")
was the first reader). 65 spurious "errors" on a full Windows pytest run.
- Database: extract _open() from __init__, add ensure_open() that
reopens iff _conn is None, and have close() null _conn after the
TRUNCATE checkpoint so re-close is idempotent.
- main.py lifespan startup: call db.ensure_open() before any setting
read, so subsequent TestClient sessions get a live connection.
- tests/storage/test_database_reopen.py: pin the four invariants —
close→ensure_open round-trips data, ensure_open is a no-op when
open, close is idempotent, and using the DB after close without
ensure_open raises (callers must opt in).
Full backend suite: 1551 pass / 1 skip / 0 errors. Ruff clean.
