Add a unique database key for code_public and provide an update path

The custom `metis` table defined in metis.install has no primary key and no unique key on `code_public`.

Right now duplicate protection is handled only in application code during import. A database-level uniqueness constraint would make the module more robust and easier to maintain on current core.

Proposed change:

- Add a unique key for `code_public` in `hook_schema()`.
- Provide an update hook for existing installs.
- Decide how to handle pre-existing duplicate rows during the update path.

Suggested acceptance criteria:

- New installs create the table with a uniqueness constraint on `code_public`.
- Existing installs can update cleanly through update hooks.
- Duplicate code insertion is prevented at the database layer.