Write integration tests for Stats Repository against Supabase

The key challenge is RLS testing: you need two Supabase JWT tokens with different `chapter_id` claims. Use Supabase's `supabase.auth.signInWithPassword` with two pre-seeded test users who have different role assignments in the database. After signing in as user-A (chapter-a scoped), all queries run through that user's RLS policies. Do not use the service role key to bypass RLS in these tests — that defeats the purpose.

For the seed script, use explicit `INSERT INTO activities (id, chapter_id, activity_date, ...)` with hardcoded UUIDs so the assertions can reference exact values. For CI, add a `supabase start` step before the test job and `supabase stop` after — document this in `supabase/README.md`. Use `flutter test integration_test/` or a dedicated test directory to keep integration tests separate from unit tests and avoid running them on every `flutter test` invocation.

This task IS the integration testing task. Structure with `setUpAll` that: (1) starts local Supabase (assume already running in CI), (2) runs seed SQL via `supabase.rpc('exec_sql', ...)` or the Supabase CLI, (3) authenticates test users via `supabase.auth.signInWithPassword`. Use `tearDownAll` to truncate seeded rows. Test groups: `group('fetchSnapshot', ...)` covering happy path, cross-chapter isolation, date boundary, empty chapter; `group('fetchPeerMentorRows', ...)` covering correct row count and sort order, RLS restriction; `group('fetchChartPoints', ...)` covering granularity bucketing.

CI must run `supabase start` before executing this test suite — document this in the test file header comment.