Implement Coordinator Stats Service

The scope abstraction is the critical design challenge. Model it as a sealed class: `sealed class StatsScope { const factory StatsScope.chapter(String chapterId) = ChapterScope; const factory StatsScope.organization(String organizationId) = OrganizationScope; }` — this is passed to the repository and mapped to the appropriate SQL WHERE clause without branching in the service. RoleAccessValidator should be async (it may need to query the database for chapter assignments). Cache key hashing: use a stable toString or toJson on StatsScope + StatsFilter period so keys are deterministic.

Donut percentage: compute raw percentages first, then apply largest-remainder method to ensure exactly 100%. For MentorStatsList, consider a separate repository method that returns pre-sorted rows to avoid in-memory sorting of large lists.

Unit tests required using flutter_test with mocked StatsRepository and RoleAccessValidator. Test cases: (1) coordinator role → scoped query called with chapter filter, (2) org-admin role → full-org query called without chapter filter, (3) DonutSegmentViewModel percentages sum exactly to 100.0 with rounding correction, (4) zero-activity types excluded from donut, (5) active mentors count uses distinct logic, (6) MentorStatsList sorted descending by activity count, (7) cache hit prevents repository call, (8) cache miss stores result, (9) Supabase error → Result.failure, (10) coordinator cannot access org-admin scope even if they pass org-admin userId. Integration test: deploy two Supabase test accounts (coordinator and org-admin) and verify scope isolation via actual RLS enforcement.