fix(test): artist_service _fixture sets artist_id (post-vii-c ArtistService reads it)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_artist_service.py

@@ -34,6 +34,9 @@ async def _fixture(db):
         path="/images/a/1.jpg", sha256="a" + "0" * 63,
         size_bytes=1, mime="image/jpeg", width=4, height=8,
         origin="downloaded", integrity_status="unknown",
+        # FC-2d-vii-c: provenance images also carry the canonical
+        # artist_id (set by importer/migration); ArtistService reads it.
+        artist_id=artist.id,
     )
     db.add(img)
     await db.flush()