Fix user_to_acl column names in 03-default-users.sql

The user_to_acl table is one of the few join tables that does NOT follow
the tablename_fieldname column naming convention — its columns are the
plain `user_id` and `acl_id`. The previous version used `user_to_acl_user_id`
and `user_to_acl_acl_id` which fail with "Unknown column".

Found by actually running bootstrap-db.sh end-to-end against a fresh DB
(was missing this verification step on the initial commit).

Verified now:
  - schema loads cleanly (36 tables)
  - seed loads (3 ACL roles, 8 email templates)
  - default users insert + ACL mapping works
  - password_verify against argon2id hashes confirms admin123 and test123

db/03-default-users.sql

@@ -19,7 +19,10 @@ VALUES
      '$argon2id$v=19$m=65536,t=4,p=1$YmxWaHpqaHRsU1dmZHFUOA$k8veR56hGcoTsciZGVYv/NuPJSyQ3wgzSTyryW/ynto',
      1);
 
--- Grant admin user superuser ACL (acl_id=1 from the acl seed data)
-INSERT INTO `user_to_acl` (`user_to_acl_user_id`, `user_to_acl_acl_id`) VALUES
+-- Grant admin user superuser ACL (acl_id=1 from the acl seed data).
+-- NOTE: user_to_acl is one of the join tables in the schema that does NOT
+-- follow the tablename_fieldname column convention — its columns are the
+-- plain `user_id` and `acl_id`.
+INSERT INTO `user_to_acl` (`user_id`, `acl_id`) VALUES
     (1, 1),  -- admin → superuser
     (2, 3);  -- testuser → user