fix(users): consider case insensitive username as unique constraint i…

…n database

In the backend it is already validated that the `username` is unique ignoring the case; now this is
also validated in the database.

infra/migrations/1725403837939_alter-table-users-username-lower-index.js

@@ -0,0 +1,18 @@
+exports.up = (pgm) => {
+  pgm.dropConstraint('users', 'users_username_key');
+
+  pgm.createIndex('users', 'LOWER(username)', {
+    unique: true,
+    name: 'users_username_lower_idx',
+  });
+};
+
+exports.down = (pgm) => {
+  pgm.dropIndex('users', 'LOWER(username)', {
+    name: 'users_username_lower_idx',
+  });
+
+  pgm.addConstraint('users', 'users_username_key', {
+    unique: ['username'],
+  });
+};