feat(attachments): PostAttachment model + 0009 (table + batch counter)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -11,6 +11,7 @@ from .import_settings import ImportSettings
 from .import_task import ImportTask
 from .ml_settings import MLSettings
 from .post import Post
+from .post_attachment import PostAttachment
 from .series_page import SeriesPage
 from .source import Source
 from .tag import Tag, TagKind, image_tag
@@ -25,6 +26,7 @@ __all__ = [
     "Source",
     "Credential",
     "Post",
+    "PostAttachment",
     "SeriesPage",
     "ImageRecord",
     "ImageProvenance",