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/image_provenance.py

@@ -0,0 +1,33 @@
+"""ImageProvenance — links an ImageRecord to a Post.
+
+Many-to-one (one image, many provenance rows) enables the enrich-on-duplicate
+rule (spec §3): when a downloaded image is a pHash dupe of an existing
+record, we append a new provenance row to the existing record rather than
+dropping the metadata.
+"""
+
+from datetime import datetime
+
+from sqlalchemy import JSON, DateTime, ForeignKey, Integer, func
+from sqlalchemy.orm import Mapped, mapped_column
+
+from .base import Base
+
+
+class ImageProvenance(Base):
+    __tablename__ = "image_provenance"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True)
+    image_record_id: Mapped[int] = mapped_column(
+        ForeignKey("image_record.id", ondelete="CASCADE"), nullable=False, index=True
+    )
+    post_id: Mapped[int] = mapped_column(
+        ForeignKey("post.id", ondelete="CASCADE"), nullable=False, index=True
+    )
+    source_id: Mapped[int] = mapped_column(
+        ForeignKey("source.id", ondelete="CASCADE"), nullable=False, index=True
+    )
+    captured_metadata: Mapped[dict | None] = mapped_column(JSON, nullable=True)
+    captured_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, server_default=func.now()
+    )