Refactor database schema for roles and visibility management

- Updated the roles table to use `code` as the primary key, enhancing readability and simplifying junction tables.
- Modified `user_roles` and `product_role_visibility` tables to reference `role_code` instead of `role_id`, maintaining many-to-many relationships.
- Added foreign key constraints and created new indexes to optimize queries.
- Removed the previous migration file that was no longer needed, ensuring a cleaner migration history.
- Updated related documentation to reflect the new schema changes and their benefits.

server/database/migrations/meta/_journal.json

@@ -15,13 +15,6 @@
       "when": 1762074397305,
       "tag": "0001_clammy_bulldozer",
       "breakpoints": true
-    },
-    {
-      "idx": 2,
-      "version": "7",
-      "when": 1762157410000,
-      "tag": "0002_refactor_roles_primary_key",
-      "breakpoints": true
     }
   ]
 }