Support literal values in owl:hasValue restrictions

[jaybea]

Problem

extract_restrictions() in rdf_parser.py silently drops any owl:hasValue restriction
where the value is a literal (e.g. owl:hasValue false, owl:hasValue "active"). Only
URIRef values are imported and stored.

This is because class_restrictions.value_uri and SnapshotRestriction.value_uri are
typed exclusively for URIs.

Current impact

Low — evrepo-core and ESEA vocabs only use URI values for hasValue. The
ontology-expressivity.ttl test fixture was adjusted to use a URI value to work around
this gap.

Scope

• rdf_parser.py:extract_restrictions() — stop skipping literal values
• class_restrictions table — add a value_literal column (or rename/generalize value_uri)
• SnapshotRestriction schema — support literal values
• skos_export_service.py — emit owl:hasValue with literal when appropriate

Context

Identified during Epic #108 Stripe 5 (PR #145).