feat: customize public profile visibility

backend/migrations/006_profile_usernames_and_visibility.sql

@@ -0,0 +1,12 @@
+ALTER TABLE users ADD COLUMN IF NOT EXISTS profile_username TEXT;
+ALTER TABLE users ADD COLUMN IF NOT EXISTS is_future_races_public BOOLEAN NOT NULL DEFAULT FALSE;
+ALTER TABLE users ADD COLUMN IF NOT EXISTS is_completed_races_public BOOLEAN NOT NULL DEFAULT FALSE;
+
+UPDATE users
+SET is_future_races_public = is_profile_public,
+    is_completed_races_public = is_profile_public
+WHERE is_profile_public = TRUE;
+
+CREATE UNIQUE INDEX IF NOT EXISTS users_profile_username_normalized_key
+  ON users (LOWER(profile_username))
+  WHERE profile_username IS NOT NULL;