fix-forum-anonymous-author down() isn't cleanly reversible

[RUKAYAT-CODER]

1791000000001-fix-forum-anonymous-author.ts:

• down() drops FK_forum_votes_authorId_users by name — but that name only exists if the FK was created with it; on dev synchronize-created or baseline schemas the FK may have a different name, making DROP CONSTRAINT IF EXISTS a silent no-op
• up()/down() purge/alter forum_votes and flag-state changes that can't be reconstructed

This matters now that migration:run/migration:revert run from scratch in CI (#1193): the revert step must not leave the DB in a broken state.

Fix: make down() resolve the FK by querying pg_constraint (not by hard-coded name), and document the irreversible parts.

[Meet-hybrid]

I would like to contribute to this issue. I have experience with JavaScript/TypeScript, React, backend APIs, testing, Git, and software architecture. I will follow the acceptance criteria, add tests, and provide a focused pull request.