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/application/test_manage_subtitles.py

@@ -41,7 +41,7 @@ from alfred.application.filesystem.manage_subtitles import (
     _to_unresolved_dto,
 )
 from alfred.domain.subtitles.entities import MediaSubtitleMetadata, SubtitleCandidate
-from alfred.domain.subtitles.services.placer import PlacedTrack, PlaceResult
+from alfred.application.subtitles.placer import PlacedTrack, PlaceResult
 from alfred.domain.subtitles.value_objects import (
     ScanStrategy,
     SubtitleFormat,

tests/application/test_subtitle_placer.py

@@ -1,4 +1,4 @@
-"""Tests for ``alfred.domain.subtitles.services.placer.SubtitlePlacer``.
+"""Tests for ``alfred.application.subtitles.placer.SubtitlePlacer``.
 
 The placer hard-links subtitle files next to a destination video, naming
 them ``{video_stem}.{lang}[.sdh|.forced].{ext}``.
@@ -22,7 +22,7 @@ from unittest.mock import patch
 import pytest
 
 from alfred.domain.subtitles.entities import SubtitleCandidate
-from alfred.domain.subtitles.services.placer import (
+from alfred.application.subtitles.placer import (
     PlacedTrack,
     PlaceResult,
     SubtitlePlacer,
@@ -198,7 +198,7 @@ class TestOSError:
         video.write_bytes(b"")
         track = _track(src)
         with patch(
-            "alfred.domain.subtitles.services.placer.os.link",
+            "alfred.application.subtitles.placer.os.link",
             side_effect=OSError("cross-device link"),
         ):
             result = placer.place([track], video)

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,