Add cache invalidation strategy to TerminologyRepository

Pass a `DateTime Function() clock` parameter defaulting to `() => DateTime.now().toUtc()` to make the staleness check fully testable. For the background refresh, use `unawaited(Future(() => _refreshInBackground(orgId)))` — import `package:meta/meta.dart` for the `@visibleForTesting` annotation on the refresh method. Avoid using Timer for the background job; a plain unawaited Future is sufficient and easier to test. The stale-while-revalidate pattern is the intended UX: users always get a fast response, and the next app session will have fresh labels.

Document the TTL default (24h) in a class-level doc comment explaining the rationale (org label changes are infrequent admin operations).

Inject a fake clock (DateTime Function() nowFn) into TerminologyRepository constructor to make TTL logic deterministic in tests. Cover: (1) fresh cache (age < TTL) → returns cache, zero Supabase calls; (2) stale cache (age >= TTL) → returns stale value immediately, background fetch fires; (3) background fetch updates cache after completion; (4) background fetch failure → cache unchanged, no exception propagated; (5) invalidateCache() → clears entry, triggers foreground fetch, returns new value; (6) invalidateCache() with network failure → returns Left(failure), cache remains empty. Use flutter_test with mockito/mocktail. All tests must be deterministic — no real Durations or actual delays.