Implement Duplicate Reviewed Flag Middleware

Implement the middleware as a Dart class following the decorator pattern: `DuplicateReviewedMiddleware implements ActivityRepository` wraps a delegate `ActivityRepository` and adds the flag logic before delegating to the wrapped save. This keeps the middleware testable in isolation without requiring a full BLoC stack. The ActivitySaveContext should be a value object (not a map) carrying typed optional fields including `duplicateReviewed: bool?`. Pitfall: using a separate UPDATE after the initial INSERT to set the flag creates a race condition if the activity is fetched between the two operations — always bundle the flag in the original upsert.

For the RLS policy, use a PostgreSQL column-level security check or a CHECK constraint combined with a role condition: `CHECK (duplicate_reviewed = false OR current_setting('role') = 'coordinator')`. Ensure the `duplicate_reviewed_at` column defaults to NULL (not a timestamp) so that unflagged activities are distinguishable from a flag set at epoch.

Unit tests using flutter_test: (1) save with duplicateReviewed=true includes duplicate_reviewed=true and duplicate_reviewed_at in the upsert payload; (2) save without duplicateReviewed context sends payload without the flag fields (no null overwrite); (3) ActivitySaveContext is correctly threaded through BLoC → repository → middleware without losing type safety; (4) middleware composes correctly when stacked with other hypothetical middleware (e.g., audit logging). Integration test against local Supabase: (1) insert an activity with duplicate_reviewed=true; (2) call check_activity_duplicates for the same peer_mentor + date + type; (3) assert the reviewed activity is excluded from results. RLS test: attempt to set duplicate_reviewed=true as a peer_mentor role user and assert PostgrestException with code 42501 (insufficient privilege).