Supabase schema migration: proxy attribution columns

Implement immutability using a PostgreSQL trigger function: `BEFORE UPDATE ON activities FOR EACH ROW EXECUTE FUNCTION prevent_attribution_update()` where the function raises an exception if NEW.registered_by != OLD.registered_by or NEW.attributed_to != OLD.attributed_to. This is more portable than CHECK constraints for immutability. For existing rows: if the table already has data, add the columns as nullable first, backfill with a sensible default (e.g., attributed_to = peer_mentor_id, registered_by = peer_mentor_id), then add the NOT NULL constraint in a second migration step to avoid locking issues on large tables. Document the two-step approach in migration comments.

Database migration tests: (1) apply migration to a clean schema and verify columns exist with correct types and constraints via information_schema queries, (2) attempt to UPDATE registered_by on an inserted row and verify it is rejected with a constraint violation, (3) attempt to UPDATE attributed_to and verify the same rejection, (4) insert a row and verify both columns accept valid auth.users UUIDs, (5) run EXPLAIN ANALYZE on a duplicate-detection query pattern to confirm index is used. These tests can be run via Supabase CLI with a local Docker instance.