final steps

backend/infra/migrations/010_add_unique_email_index.sql

@@ -0,0 +1,20 @@
+-- Migration 010: enforce unique emails in users
+-- Matches runtime expectation for idx_users_email_unique.
+
+BEGIN;
+
+DO $$
+BEGIN
+  IF EXISTS (
+    SELECT 1
+    FROM users
+    GROUP BY email
+    HAVING COUNT(*) > 1
+  ) THEN
+    RAISE EXCEPTION 'Cannot create unique index idx_users_email_unique: duplicate emails exist in users';
+  END IF;
+END $$;
+
+CREATE UNIQUE INDEX IF NOT EXISTS idx_users_email_unique ON users(email);
+
+COMMIT;