H-60: harden Captain migration rollback and concurrency (#10)

Co-authored-by: Rogee <rogee@ipao.vip>

backend/migrations/000079_make_captain_bindings_unique.down.sql

@@ -1,4 +1,7 @@
-DROP INDEX IF EXISTS idx_captain_inboxes_active_inbox;
-DROP INDEX IF EXISTS idx_agent_bot_inboxes_bot_inbox;
-DROP INDEX IF EXISTS idx_agent_bots_captain_assistant;
-ALTER TABLE agent_bots DROP COLUMN IF EXISTS captain_assistant_id;
+-- The up migration merges duplicate bots/bindings and rewrites references.
+-- Those deleted rows and original references cannot be reconstructed safely.
+DO $$
+BEGIN
+  RAISE EXCEPTION 'migration 000079 is irreversible: restore from backup instead of migrating down';
+END
+$$;