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/domain/test_subtitle_matcher.py

@@ -18,7 +18,7 @@ from __future__ import annotations
 
 import pytest
 
-from alfred.domain.subtitles.entities import SubtitleCandidate
+from alfred.domain.subtitles.entities import SubtitleScanResult
 from alfred.domain.subtitles.services.matcher import SubtitleMatcher
 from alfred.domain.subtitles.value_objects import (
     SubtitleFormat,
@@ -40,8 +40,8 @@ def _track(
     stype: SubtitleType = SubtitleType.STANDARD,
     confidence: float = 1.0,
     is_embedded: bool = False,
-) -> SubtitleCandidate:
-    return SubtitleCandidate(
+) -> SubtitleScanResult:
+    return SubtitleScanResult(
         language=lang,
         format=fmt,
         subtitle_type=stype,