Replace the in-memory InMemoryStore with a SQLAlchemy-backed
CycloneStore that persists Batch, Claim, Remittance, Match, and
ActivityEvent rows to a configurable DB engine (sqlite by default,
overridable via CYCLONE_DB_URL).
Public API preserved: add / get_batch / iter_claims / iter_remittances
/ distinct_providers / recent_activity. New T10/T12 stubs:
list_unmatched, manual_match, manual_unmatch.
Backward-compat shims for tests that called ._batches.clear() or
acquired ._lock as a context manager.
Idempotency: add() does a per-row session.get(Claim, c.id) (and
s.get(Remittance, ...)) check before each insert; duplicates are
skipped with a warning. This makes re-uploading the same fixture
idempotent instead of raising IntegrityError on the PK.
Auto-init DB fixture (backend/tests/conftest.py) sets
CYCLONE_DB_URL to a per-test sqlite file and calls db.init_db()
once per test, replacing the old module-scoped in-memory fixture.
Tyler