feat(dimensional): add dim_certificate_type to normalize certificate type hierarchy

[blarghmatey]

Context

PR #1868 added tfact_certificate which includes inline certificate type classification logic. Reviewer suggested this should be extracted into a dim_certificate_type dimension.

What

• Create dim_certificate_type with surrogate key, type code, display label, and hierarchy (e.g., honor/verified/audit groupings)
• Move certificate type classification logic out of tfact_certificate and into the dimension
• Add certificate_type_fk to tfact_certificate

Current State

Certificate type logic currently lives in tfact_certificate.sql (see type classification CTE). The fact table contains certificate_type as a raw string.

Acceptance Criteria

• dim_certificate_type model created with stable surrogate key
• _dim_certificate_type.yml with PK uniqueness and accepted_values tests
• certificate_type_fk added to tfact_certificate
• Existing certificate_type string column kept for backward compatibility or migration notes added

[blarghmatey]

Closing as implemented. The recent dimensional model work fully addressed all acceptance criteria:

• ✅ dim_certificate_type model created in src/ol_dbt/models/dimensional/dim_certificate_type.sql with a stable integer surrogate key and rows for: verified, professional, completion, audit, micromasters, honor, credit
• ✅ _reference_dimensions.yml includes PK uniqueness, not_null, and column documentation for dim_certificate_type
• ✅ certificate_type_fk column added to tfact_certificate with FK relationship test to dim_certificate_type.certificate_type_pk
• ✅ Existing certificate_type_code string column kept for backward compatibility