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.

alfred/domain/shared/ports/__init__.py

@@ -7,11 +7,13 @@ Protocol without going through real I/O.
 """
 
 from .filesystem_scanner import FileEntry, FilesystemScanner
+from .language_repository import LanguageRepository
 from .media_prober import MediaProber, SubtitleStreamInfo
 
 __all__ = [
     "FileEntry",
     "FilesystemScanner",
+    "LanguageRepository",
     "MediaProber",
     "SubtitleStreamInfo",
 ]

alfred/domain/shared/ports/language_repository.py

@@ -0,0 +1,36 @@
+"""LanguageRepository port — abstracts canonical language lookup.
+
+The adapter (typically loading from ISO 639 YAML knowledge) maps a wide
+range of raw forms (codes, English/native names, aliases) onto the
+canonical :class:`Language` value object. Domain code accepts the port
+via constructor injection; tests can pass a small in-memory fake.
+"""
+
+from __future__ import annotations
+
+from typing import Protocol
+
+from alfred.domain.shared.value_objects import Language
+
+
+class LanguageRepository(Protocol):
+    """Canonical language lookup."""
+
+    def from_iso(self, code: str) -> Language | None:
+        """Look up by canonical ISO 639-2/B code (case-insensitive)."""
+        ...
+
+    def from_any(self, raw: str) -> Language | None:
+        """Look up by any known representation: ISO code, name, alias.
+
+        Case-insensitive. Returns ``None`` when the raw form is unknown.
+        """
+        ...
+
+    def all(self) -> list[Language]:
+        """Return all known languages, in a stable order."""
+        ...
+
+    def __contains__(self, raw: str) -> bool: ...
+
+    def __len__(self) -> int: ...