Allow configurable column types for auditable_id and associated_id

[ryush00]

Follow-up to #361: Auditing models with UUID pk

Currently, only the result of calling to_i on a UUID string is stored, which gives the illusion of working correctly:

> "78035962-1234-1234-1234-9c394a3e5a0b".to_i
=> 78035962

• When the UUID contains hyphens, to_i stops at the first alphabet.
• If there are no hyphens, the integer may exceed the 4‑byte limit and raise an error.

To address this, I propose one of the following:

1. Update the README to explicitly state that UUIDs are not supported.
2. Extend PR Support UUID primary keys for User #333’s column‑type configuration to include auditable_id and associated_id, allowing full UUID support.
3. Convert all ID columns to string types so they can store complete UUIDs.

PR #333 currently only lets you change user_id’s type. Extending support to these other ID columns would enable full UUID compatibility.

Related: #651