Implement reactive flag change propagation

Implement using Riverpod's `StateNotifier` or `Notifier` (Riverpod 2.x). The provider watches a `StreamProvider` backed by the Supabase Realtime channel for the `feature_flags` table scoped to the current organization. On stream event, parse the changed flag, recompute only the affected entry in the resolved map (immutable copy with updated entry using `Map.from(...)..update(...)`), and set state to the new map. For selective rebuilds, expose individual flag providers via `provider.select((map) => map[flagKey])` so each FeatureGate widget only re-renders when its specific flag changes.

For org switching, listen to an `activeOrganizationProvider` and use `ref.watch` to auto-dispose and re-create the flag stream when the org changes. Add a `StaleCacheNotifier` bool to signal UI when a reactive update failed.

Unit and integration tests using flutter_test. Test cases: (1) Mock repository emits a change event → verify provider rebuilds and isEnabled returns updated value; (2) Mock org switch event → verify map is cleared and re-initialized with new org's flags; (3) Simulate rapid 5 consecutive toggles within 100ms → verify only 1 or 2 rebuilds occur (debounce working); (4) Mock network failure during reactive update → verify stale values retained and no exception thrown; (5) Widget test: wrap a FeatureGate widget in a ProviderScope, simulate flag toggle via mock repository, assert widget content changes without app restart. Use flutter_test WidgetTester for widget-level tests.