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/post.py

@@ -0,0 +1,33 @@
+"""Post — provenance anchor for content downloaded from a Source.
+
+A Post is one creator post; it may contain many images/videos.
+"""
+
+from datetime import datetime
+
+from sqlalchemy import JSON, DateTime, ForeignKey, Integer, String, Text, UniqueConstraint, func
+from sqlalchemy.orm import Mapped, mapped_column
+
+from .base import Base
+
+
+class Post(Base):
+    __tablename__ = "post"
+    __table_args__ = (
+        UniqueConstraint("source_id", "external_post_id", name="uq_post_source_external_id"),
+    )
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True)
+    source_id: Mapped[int] = mapped_column(
+        ForeignKey("source.id", ondelete="CASCADE"), nullable=False, index=True
+    )
+    external_post_id: Mapped[str] = mapped_column(String(128), nullable=False)
+    post_url: Mapped[str | None] = mapped_column(Text, nullable=True)
+    post_title: Mapped[str | None] = mapped_column(Text, nullable=True)
+    post_date: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
+
+    raw_metadata: Mapped[dict | None] = mapped_column(JSON, nullable=True)
+
+    downloaded_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, server_default=func.now()
+    )