feat(series): SeriesPage model + 0005 migration (unique image, ordered pages)

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 .series_page import SeriesPage
 from .source import Source
 from .tag import Tag, TagKind, image_tag
 from .tag_alias import TagAlias
@@ -24,6 +25,7 @@ __all__ = [
     "Source",
     "Credential",
     "Post",
+    "SeriesPage",
     "ImageRecord",
     "ImageProvenance",
     "Tag",