A factory over an injectable backend, so persistence can be tested without
a DOM.
Fixes the unguarded JSON.parse that would white-screen the app on one bad
value, and falls back to memory when localStorage itself throws (Safari
private mode, full quota).
migrateLegacyKeys converts old "Sat Aug 15 2026" keys in place. It's
idempotent, and it leaves anything it can't parse alone rather than
deleting it.
Aculix Technologies