Prepare secure config and reliable workout sync

backend/migrations/002_workout_idempotency.sql

@@ -0,0 +1,7 @@
+-- Preserve offline workout identity across network retries.
+-- Apply once to an existing database created from the original schema.sql.
+-- Fresh databases created from the updated schema.sql already contain this column.
+
+ALTER TABLE workouts
+    ADD COLUMN client_workout_uuid CHAR(36) NULL AFTER user_id,
+    ADD UNIQUE KEY uq_user_client_workout (user_id, client_workout_uuid);