feat: implement user linking for members and update database schema for user associations

backend/migrations/20260722_add_member_user_link.sql

@@ -0,0 +1,4 @@
+ALTER TABLE member
+  ADD COLUMN user_id INT NULL,
+  ADD UNIQUE KEY member_user_id_unique (user_id),
+  ADD CONSTRAINT member_user_id_fk FOREIGN KEY (user_id) REFERENCES user(id) ON DELETE SET NULL;