refactor(subtitles): move SubtitlePlacer to application layer

The placer performs filesystem I/O (os.link) — it belongs in the
application layer, not the domain. Domain services should be pure.

- Move alfred/domain/subtitles/services/placer.py to
  alfred/application/subtitles/placer.py
- Move tests/domain/test_subtitle_placer.py to
  tests/application/test_subtitle_placer.py
- Update all callers (manage_subtitles use case, metadata store, tests)
- Drop placer re-exports from domain.subtitles.services.__init__

tests/infrastructure/test_subtitle_metadata_store.py

@@ -17,7 +17,7 @@ from __future__ import annotations
 from pathlib import Path
 
 from alfred.domain.subtitles.entities import SubtitleCandidate
-from alfred.domain.subtitles.services.placer import PlacedTrack
+from alfred.application.subtitles.placer import PlacedTrack
 from alfred.domain.subtitles.value_objects import (
     SubtitleFormat,
     SubtitleLanguage,