fix(database): restore cascade delete for study dependencies

[ibrahimozkn]

Summary

• restore ON DELETE CASCADE on the study foreign keys used by repo, study_fitbit_credentials, study_invite, and study_subject
• add a forward migration to safely update existing instances
• align database/studyu-schema.sql and add a regression test covering study deletion cleanup

Testing

Self-hosted Supabase

1. Start the local Supabase stack:

   supabase start

2. Apply the latest database state from this branch on a fresh local instance:

   supabase db reset

3. Run the SQL test suite, including the new cascade regression coverage:

   supabase test db

Expected result

• deleting a row from public.study also deletes dependent rows in public.repo, public.study_fitbit_credentials, public.study_invite, and public.study_subject
• the regression test in supabase/tests/010-study.sql passes

[johannesvedder]

The description you added will not work by default when using the seed.sql Use seed-ci.sql for testing to let the supabase tests pass.
with seed.sql

With seed-ci.sql as seed.sql