fix(gallery+tests): alias Post inside artist EXISTS; tests stop asserting synthetic Source

gallery_service._provenance_clause artist branch was correlating its bare
Post reference to the outer query's primary_post_id outer-join, so the
artist filter silently matched zero rows for images with no primary post.
Alias Post inside the EXISTS subquery so SQLAlchemy adds it to the inner
FROM rather than treating it as a correlated outer table.

Five sidecar/import tests still asserted that a synthetic Source row
appears after a filesystem import. Alembic 0030 retired that behavior;
the Post sits null-source and the artist linkage lives on Post.artist_id.
Updated test_sidecar_creates_provenance, test_reimport_same_post_idempotent,
test_sidecar_artist_used_when_no_folder_artist, test_supersede_applies_new_file_sidecar,
and test_apply_sidecar_recovers_from_integrity_error to assert
post.source_id IS NULL + post.artist_id linkage instead.

tests/test_importer_provenance_race.py

@@ -29,7 +29,6 @@ from backend.app.models import (
     ImageProvenance,
     ImageRecord,
     ImportSettings,
-    Source,
 )
 from backend.app.services.importer import Importer
 from backend.app.services.thumbnailer import Thumbnailer
@@ -139,9 +138,11 @@ def test_apply_sidecar_recovers_from_integrity_error(
     r = importer.import_one(m)
     assert r.status == "imported"
     rec = importer.session.get(ImageRecord, r.image_id)
-    src = importer.session.execute(select(Source)).scalar_one()
+    # alembic 0030 stopped creating synthetic Source rows for filesystem
+    # sidecars; the Post sits null-source and the provenance row points at
+    # it directly. The race-recovery path tested below operates on
+    # ImageProvenance regardless of Source presence.
     assert rec is not None
-    assert src is not None
 
     # Monkeypatch session.execute so the FIRST select inside _apply_sidecar's
     # existence-check returns a "no row" wrapper. Subsequent selects (e.g.