auto-populate missing username field with email local-part

prisma/migrations/20260317103000_make_username_required_unique/migration.sql

@@ -0,0 +1,27 @@
+UPDATE users
+SET username = lower(split_part(email, '@', 1))
+WHERE username IS NULL OR btrim(username) = '';
+
+WITH ranked_users AS (
+  SELECT id,
+         username,
+         row_number() OVER (PARTITION BY username ORDER BY created_at, id) AS rn
+  FROM users
+),
+deduplicated AS (
+  SELECT id,
+         CASE
+           WHEN rn = 1 THEN username
+           ELSE left(username, greatest(1, 24 - char_length(rn::text))) || rn::text
+         END AS next_username
+  FROM ranked_users
+)
+UPDATE users u
+SET username = d.next_username
+FROM deduplicated d
+WHERE u.id = d.id;
+
+ALTER TABLE users
+ALTER COLUMN username SET NOT NULL;
+
+CREATE UNIQUE INDEX users_username_key ON users (username);