Implement Core Hierarchy Service

Implement as a Riverpod-managed service class HierarchyService injected via a Provider. Internal dependencies: OrganizationUnitRepository (data layer), HierarchyStructureValidator (task-005), and HierarchyCache (in-memory LRU cache keyed by organizationId). The repository handles raw Supabase queries; the service handles business rules. Use a recursive CTE Postgres function (get_organization_tree) invoked via supabase.rpc() to fetch the full tree in one round-trip — avoid assembling the tree client-side from a flat list.

For moveNode, fetch the ancestor path of the proposed new parent before calling the validator (one additional query). For Realtime subscriptions, subscribe to the organization_units table filtered by organization_id and pipe changes through a StreamController exposed as watchSubtree(). The cache should store the last fetched tree per organizationId and be invalidated on any write operation to force a fresh fetch on next read. NHF's 1400-chapter structure makes caching especially important for the admin portal's performance.

Unit tests with mocked OrganizationUnitRepository covering all service methods including edge cases: moving a node to its own descendant (circular), deleting a node with children, renaming to a name that conflicts with a sibling. Integration tests against a local Supabase instance for: tree load with 50+ nodes, concurrent write conflict handling, Realtime stream emission on update. Use flutter_test and mockito for unit tests. Test coverage target: 90% of service class lines.

Performance test: load NHF-scale mock tree (1400 nodes) and assert response within 1.5s.