feat(ml): presentation auto-hide settings + review table (#141 step 3)

MLSettings gains presentation_auto_apply_enabled / _threshold (default 0.90) +
presentation_conflict_threshold (default 0.50): banner/editor auto-hide with a
FLAT threshold (decoupled from content-head graduation), plus the "also looks
like content" conflict cut. New presentation_review table (image, presentation
tag, conflict tag + score, created/resolved_at) records auto-hides flagged for
review. Migration 0082 (columns + table), ml_admin API (editable + get_settings
+ _validate bounds), settings roundtrip/bounds test. The sweep that reads these
knobs + the Settings UI land in step 4.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

backend/app/models/__init__.py

@@ -28,6 +28,7 @@ from .pixiv_failed_media import PixivFailedMedia
 from .pixiv_seen_media import PixivSeenMedia
 from .post import Post
 from .post_attachment import PostAttachment
+from .presentation_review import PresentationReview
 from .series_chapter import SeriesChapter
 from .series_page import SeriesPage
 from .series_suggestion import SeriesSuggestion
@@ -57,6 +58,7 @@ __all__ = [
     "SubscribeStarSeenMedia",
     "Post",
     "PostAttachment",
+    "PresentationReview",
     "SeriesChapter",
     "SeriesPage",
     "SeriesSuggestion",