- Add SetSubscriptionUpdatedAt to Store interface, remove DB() accessor
- Rename store files to store_sqlite.go and store_postgres.go
- Use camelCase for test function names
- Add tests for upsert field updates and multi-user removal
- Use transaction in setupNewPostgresDB
- Use lowercase "excluded." in PostgreSQL upsert query
Share test logic in store_test.go with thin per-backend wrappers.
Add SetSubscriptionUpdatedAt to both stores, removing the need for
raw SQL and the DB() accessor in tests.
This adds the detection of `NTFY_PASSWORD_HASH` when creating a user or
changing its passsword so that scripts don't have to manipulate the bare
password.
