refactor: align backend layout and supabase infra

Consolidate backend modules/tests under the backend package while syncing Supabase compose/env config and related plans.

infra/docker/volumes/db/jwt.sql

@@ -0,0 +1,4 @@
+\set jwt_secret `echo "$JWT_SECRET"`
+\set jwt_exp `echo "$JWT_EXP"`
+ALTER DATABASE postgres SET "app.settings.jwt_secret" TO :'jwt_secret';
+ALTER DATABASE postgres SET "app.settings.jwt_exp" TO :'jwt_exp';