Mirror the MediaProber / FilesystemScanner pattern for language lookup:
- New Protocol `LanguageRepository` in alfred.domain.shared.ports
covering from_iso, from_any, all, __contains__, __len__ — the
surface previously coupled to the concrete LanguageRegistry.
- SubtitleKnowledgeBase types its `language_registry` parameter
against the Protocol; the concrete LanguageRegistry stays in
infrastructure as the YAML-backed adapter and remains the default
when no repository is injected.
- New unit tests in tests/infrastructure/test_language_registry.py
cover the adapter surface (from_iso, from_any, membership,
case-insensitivity, non-string inputs).
Behaviour is unchanged for existing callers. The split opens the
door to in-memory fakes in future tests without loading the full
ISO 639 YAML.
