Write unit and integration tests for repositories

Use mocktail (preferred over mockito for null-safe Dart) to mock SupabaseClient and its chain calls (from().select(), rpc()). For mocking the stream in DuplicateQueueRepository tests, use a StreamController>>() and emit events manually to simulate Realtime updates. For the timeout test in DuplicateCheckRepository, wrap the repository method call in a fake async zone using fake_async package so the test runs instantly without real delays. For integration tests, use a dedicated supabase/seed.sql that creates test chapters, users with different roles, and sample activities.

The EXPLAIN ANALYZE index test should query: SELECT * FROM activities WHERE chapter_id = $1 AND peer_mentor_id = $2 AND activity_date = $3 and parse the query plan for 'Index Scan' — fail the test if 'Seq Scan' appears on a table with more than 1000 rows.

This task IS the testing task. Structure tests in three groups: (1) Unit tests — mock all Supabase calls using mocktail's when/thenAnswer pattern; run with flutter test; (2) Integration tests — use supabase_flutter connected to a local Supabase Docker instance started via supabase start; seed test data in setUp() and clean up in tearDown(); (3) RLS policy tests — run as SQL scripts via supabase db test or as Dart integration tests using two SupabaseClient instances authenticated with different test JWTs. CI pipeline should run unit tests on every PR and integration tests on merge to main. Document the local Supabase setup steps in a TESTING.md comment block at the top of the integration test file.