refactor(language): LanguageRepository port + SubtitleKnowledgeBase wired to it

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.

CHANGELOG.md

@@ -15,8 +15,23 @@ callers).
 
 ## [Unreleased]
 
+### Added
+
+- **`LanguageRepository` port** in `alfred.domain.shared.ports`. Structural
+  Protocol covering `from_iso`, `from_any`, `all`, `__contains__`, `__len__`
+  — the surface previously coupled to the concrete `LanguageRegistry`.
+  Mirrors the `MediaProber` / `FilesystemScanner` pattern: domain code
+  depends on the Protocol, infrastructure provides the YAML-backed
+  adapter. Tests in `tests/infrastructure/test_language_registry.py`.
+
 ### Internal
 
+- **`SubtitleKnowledgeBase` types `language_registry` against the
+  `LanguageRepository` port** instead of the concrete `LanguageRegistry`
+  class. The default constructor still instantiates the concrete adapter
+  when no repository is injected — behaviour is unchanged for existing
+  callers. Opens the door to in-memory fakes in future tests without
+  loading the full ISO 639 YAML.
 - **Moved `detect_media_type` and `enrich_from_probe` from
   `alfred.application.filesystem` to `alfred.application.release`**.
   They are inspection-pipeline helpers — their natural home is next to