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/download_event.py

@@ -0,0 +1,28 @@
+"""DownloadEvent — log of every gallery-dl run, populated by FC-3."""
+
+from datetime import datetime
+
+from sqlalchemy import BigInteger, DateTime, ForeignKey, Integer, String, Text, func
+from sqlalchemy.orm import Mapped, mapped_column
+
+from .base import Base
+
+
+class DownloadEvent(Base):
+    __tablename__ = "download_event"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True)
+    source_id: Mapped[int] = mapped_column(
+        ForeignKey("source.id", ondelete="CASCADE"), nullable=False, index=True
+    )
+    post_id: Mapped[int | None] = mapped_column(
+        ForeignKey("post.id", ondelete="SET NULL"), nullable=True, index=True
+    )
+    status: Mapped[str] = mapped_column(String(32), nullable=False)  # pending|running|ok|error
+    started_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, server_default=func.now()
+    )
+    finished_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
+    bytes_downloaded: Mapped[int] = mapped_column(BigInteger, nullable=False, default=0)
+    files_count: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+    error: Mapped[str | None] = mapped_column(Text, nullable=True)