fix: wipe user secrets when user is soft-deleted (#24985)

Extend the delete_deleted_user_resources() trigger so that secrets
belonging to a soft-deleted user are removed in the same transaction as
the existing api_keys and user_links cleanup.

user_secrets.user_id has ON DELETE CASCADE, but Coder soft-deletes users
by flipping users.deleted rather than removing the row, so the foreign key
cascade never fires and secrets would otherwise survive deletion.

Assisted by Coder Agents.

coderd/database/migrations/000490_trigger_delete_user_secrets.down.sql

@@ -0,0 +1,27 @@
+-- Drop the BEFORE INSERT/UPDATE guard added by 000489.
+DROP TRIGGER IF EXISTS trigger_upsert_user_secrets ON user_secrets;
+DROP FUNCTION IF EXISTS insert_user_secret_fail_if_user_deleted;
+
+-- Restore the previous body of delete_deleted_user_resources() from
+-- 000194_trigger_delete_user_user_link.up.sql, dropping the
+-- user_secrets cleanup added by 000489.
+CREATE OR REPLACE FUNCTION delete_deleted_user_resources() RETURNS trigger
+	LANGUAGE plpgsql
+AS $$
+DECLARE
+BEGIN
+	IF (NEW.deleted) THEN
+		-- Remove their api_keys
+		DELETE FROM api_keys
+		WHERE user_id = OLD.id;
+
+		-- Remove their user_links
+		-- Their login_type is preserved in the users table.
+		-- Matching this user back to the link can still be done by their
+		-- email if the account is undeleted. Although that is not a guarantee.
+		DELETE FROM user_links
+		WHERE user_id = OLD.id;
+	END IF;
+	RETURN NEW;
+END;
+$$;