refactor(subtitles): inject MediaProber/FilesystemScanner ports into domain services

Domain services no longer call subprocess or pathlib directly. Introduces
two Protocol ports in domain/shared/ports/:

  MediaProber.list_subtitle_streams(video) -> list[SubtitleStreamInfo]
  FilesystemScanner.scan_dir / stat / read_text  -> list[FileEntry] | ...

Concrete adapters live in infrastructure/:

  FfprobeMediaProber          (wraps subprocess + ffprobe + JSON)
  PathlibFilesystemScanner    (wraps pathlib + os reads)

SubtitleIdentifier and PatternDetector now take (kb, prober, scanner) at
construction time. Their internals work over FileEntry snapshots and
SubtitleStreamInfo records — no more ad-hoc Path.is_file/iterdir/stat or
embedded subprocess.run loops. _count_entries now takes raw SRT text
(returned by scanner.read_text) so SRT-only entry counting stays out of
the FS layer.

manage_subtitles use case instantiates the two adapters once and injects
them into both services. Tests pass real adapters and patch
`alfred.infrastructure.probe.ffprobe_prober.subprocess.run` for the
ffprobe-failure cases. _classify_single tests build FileEntry via a
small helper.

Domain is now free of subprocess / direct filesystem reads in the
subtitle pipeline. The only remaining I/O hooks are FilePath VO
convenience methods (exists/is_file/is_dir) which stay as a deliberate
affordance on the value object.

tests/domain/test_subtitle_pattern_detector.py

@@ -25,8 +25,10 @@ from unittest.mock import patch
 
 import pytest
 
-from alfred.infrastructure.knowledge.subtitles.base import SubtitleKnowledgeBase
 from alfred.domain.subtitles.services.pattern_detector import PatternDetector
+from alfred.infrastructure.filesystem.scanner import PathlibFilesystemScanner
+from alfred.infrastructure.knowledge.subtitles.base import SubtitleKnowledgeBase
+from alfred.infrastructure.probe.ffprobe_prober import FfprobeMediaProber
 
 
 @pytest.fixture(scope="module")
@@ -36,7 +38,7 @@ def kb():
 
 @pytest.fixture
 def detector(kb):
-    return PatternDetector(kb)
+    return PatternDetector(kb, FfprobeMediaProber(), PathlibFilesystemScanner())
 
 
 def _make_video(folder: Path, name: str = "Show.S01E01.mkv") -> Path: