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/source.py

@@ -0,0 +1,31 @@
+"""Source — a platform-specific URL owned by an Artist (e.g., a Patreon URL).
+
+Multiple sources per artist support creators with cross-platform presence.
+"""
+
+from datetime import datetime
+
+from sqlalchemy import JSON, Boolean, DateTime, ForeignKey, Integer, String, Text
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from .base import Base
+
+
+class Source(Base):
+    __tablename__ = "source"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True)
+    artist_id: Mapped[int] = mapped_column(
+        ForeignKey("artist.id", ondelete="CASCADE"), nullable=False, index=True
+    )
+    platform: Mapped[str] = mapped_column(String(64), nullable=False)
+    url: Mapped[str] = mapped_column(Text, nullable=False)
+    enabled: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True)
+
+    config_overrides: Mapped[dict | None] = mapped_column(JSON, nullable=True)
+
+    last_checked_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
+    last_error: Mapped[str | None] = mapped_column(Text, nullable=True)
+    check_interval_override: Mapped[int | None] = mapped_column(Integer, nullable=True)
+
+    artist = relationship("Artist", back_populates="sources")