refactor(subtitles): rename SubtitleCandidate → SubtitleScanResult

The old name conflated 'might become a placed subtitle' with 'what a
scan pass produced'. The class is the output of a scan/identify pass —
language/format may still be None while classification is in progress,
confidence reflects classifier certainty, raw_tokens holds filename
fragments under analysis. SubtitleScanResult says that directly.

Pure rename + refreshed docstring; no behavior change. Touches the
domain entity, the matcher/identifier/utils services, the
manage_subtitles use case, the placer, the metadata store, the
shared-media cross-ref comment, and 7 test modules.

tests/application/test_subtitle_placer.py

@@ -21,7 +21,7 @@ from unittest.mock import patch
 
 import pytest
 
-from alfred.domain.subtitles.entities import SubtitleCandidate
+from alfred.domain.subtitles.entities import SubtitleScanResult
 from alfred.application.subtitles.placer import (
     PlacedTrack,
     PlaceResult,
@@ -46,8 +46,8 @@ def _track(
     fmt=SRT,
     stype=SubtitleType.STANDARD,
     is_embedded: bool = False,
-) -> SubtitleCandidate:
-    return SubtitleCandidate(
+) -> SubtitleScanResult:
+    return SubtitleScanResult(
         language=lang,
         format=fmt,
         subtitle_type=stype,