feat: define unified schema (Artist, Source, Post, ImageRecord, etc.) and initial migration

Implements the data model from spec §3 in one go so FC-2/FC-3 don't need
schema-adding migrations of their own. Artist is the unified entity for
both gallery 'artist:' tags and GallerySubscriber Subscriptions
(is_subscription flag). ImageProvenance is many-to-one, enabling the
enrich-on-duplicate rule for downloaded content that pHash-matches an
existing record.

The SigLIP embedding column uses pgvector(1152) for SigLIP-so400m;
swapping models in FC-2 will require a column-width migration.

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

backend/app/models/__init__.py

@@ -1,7 +1,24 @@
 """All ORM models. Import this module to make every model visible to Alembic."""
 
+from .artist import Artist
 from .base import Base
+from .credential import Credential
+from .download_event import DownloadEvent
+from .image_provenance import ImageProvenance
+from .image_record import ImageRecord
+from .post import Post
+from .source import Source
+from .tag import Tag, image_tag
 
-# Concrete models land in Task 6 and re-export here.
-
-__all__ = ["Base"]
+__all__ = [
+    "Base",
+    "Artist",
+    "Source",
+    "Credential",
+    "Post",
+    "ImageRecord",
+    "ImageProvenance",
+    "Tag",
+    "image_tag",
+    "DownloadEvent",
+]