feat(members): split username/display_name, sync updates to authentik, add password reset API and refresh docs

backend/scripts/migrate_add_users_username.sql

@@ -0,0 +1,16 @@
+ALTER TABLE users
+  ADD COLUMN IF NOT EXISTS username TEXT;
+
+DO $$
+BEGIN
+  IF NOT EXISTS (
+    SELECT 1
+    FROM pg_constraint
+    WHERE conname = 'uq_users_username'
+  ) THEN
+    ALTER TABLE users
+      ADD CONSTRAINT uq_users_username UNIQUE (username);
+  END IF;
+END $$;
+
+CREATE INDEX IF NOT EXISTS idx_users_username ON users(username);