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>

tests/test_models.py

@@ -0,0 +1,29 @@
+"""Smoke test — every model imports cleanly and shares the same metadata."""
+
+from backend.app.models import (
+    Artist,
+    Base,
+    ImageRecord,
+)
+
+
+def test_all_models_use_shared_metadata():
+    expected_tables = {
+        "artist",
+        "source",
+        "credential",
+        "post",
+        "image_record",
+        "image_provenance",
+        "tag",
+        "image_tag",
+        "download_event",
+    }
+    assert expected_tables.issubset(Base.metadata.tables.keys())
+
+
+def test_artist_has_subscription_flag():
+    # Sanity: the unified data model decision (Subscription IS Artist) shows up
+    # as a boolean flag on Artist, per spec §3.
+    assert "is_subscription" in {c.name for c in Artist.__table__.columns}
+    assert "primary_post_id" in {c.name for c in ImageRecord.__table__.columns}