-- Make username uniqueness case-insensitive.
--
-- The 0001 schema declared `username text NOT NULL UNIQUE`, which let
-- "Alice" and "alice" both register and then race for who gets which
-- session cookie on subsequent logins. Adding a partial / functional
-- unique index over `lower(username)` blocks the conflict at the DB
-- layer regardless of how the application normalises the input.
--
-- The original `username UNIQUE` constraint is kept — it now overlaps
-- with the new index but the duplication is cheap and removing the
-- inline constraint would require a multi-step destructive migration
-- on existing data.

CREATE UNIQUE INDEX users_username_lower_uniq
    ON users (lower(username));