Add a nullable `description` column to the credential entity and accept it
on the internal credentials API. A user can now record what a credential is
for and where it belongs.
The column is `text`. The 512-character cap lives in `@n8n/api-types` and
applies in the DTO and the entity validator. A later reader must agree its
truncation limit with this cap.
The value is trimmed on write. A blank value is stored as `null`.
The migration uses a raw `ALTER TABLE`. Several tables hold cascading
foreign keys to `credentials_entity`, so the SQLite table recreation that
`addColumns` triggers would delete their rows.
The public API contract is unchanged.
