feat: backend service

backend/internal/postgres/migrations/00002_auth_challenge_locale.sql

@@ -0,0 +1,13 @@
+-- +goose Up
+-- Persist the locale captured at send-email-code so it can be replayed at
+-- confirm-email-code when the auth flow needs `preferred_language` to seed
+-- a freshly-created `accounts` row. Existing rows default to '' and are
+-- treated by the auth service as "no captured locale", in which case the
+-- service falls back to the geoip-derived language and finally to "en".
+
+ALTER TABLE backend.auth_challenges
+    ADD COLUMN preferred_language text NOT NULL DEFAULT '';
+
+-- +goose Down
+ALTER TABLE backend.auth_challenges
+    DROP COLUMN preferred_language;