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_sidecar_import.py

@@ -75,9 +75,15 @@ def test_sidecar_creates_provenance(importer, import_layout):
     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()
     post = importer.session.execute(select(Post)).scalar_one()
-    assert src.platform == "patreon"
+    # Filesystem-imported sidecar posts no longer create a synthetic Source
+    # (alembic 0030 / nullable post.source_id refactor). The Post is linked
+    # to the artist via Post.artist_id; Post.source_id stays NULL until a
+    # real subscription for the (artist, platform) gets added.
+    assert post.source_id is None
+    assert importer.session.execute(
+        select(func.count()).select_from(Source)
+    ).scalar_one() == 0
     assert post.external_post_id == "555"
     assert post.post_url == "https://patreon.com/posts/555"
     assert post.post_title == "Set 1"
@@ -106,9 +112,11 @@ def test_reimport_same_post_idempotent(importer, import_layout):
     _sidecar(m2, payload)
     r2 = importer.import_one(m2)
     assert r2.status == "imported"
+    # No synthetic Source after alembic 0030; both imports still resolve to
+    # a single null-source Post (deduped by uq_post_artist_external_id_null_source).
     assert importer.session.execute(
         select(func.count()).select_from(Source)
-    ).scalar_one() == 1
+    ).scalar_one() == 0
     assert importer.session.execute(
         select(func.count()).select_from(Post)
     ).scalar_one() == 1
@@ -164,5 +172,11 @@ def test_sidecar_artist_used_when_no_folder_artist(importer, import_layout):
     a = importer.session.execute(
         select(Artist).where(Artist.slug == "yuki")
     ).scalar_one()
-    src = importer.session.execute(select(Source)).scalar_one()
-    assert src.artist_id == a.id
+    # No synthetic Source after alembic 0030; the artist linkage lives on
+    # Post.artist_id (NOT NULL FK).
+    post = importer.session.execute(select(Post)).scalar_one()
+    assert post.artist_id == a.id
+    assert post.source_id is None
+    assert importer.session.execute(
+        select(func.count()).select_from(Source)
+    ).scalar_one() == 0