test(series): assert group start_page, not per-page stated_page

add_post now stamps the post's parsed START (constant) on every staged
pending page so the group start survives junk removal; list_pages
surfaces it as start_page. Update the stale per-page [9,10,11] assertion
to check grp["start_page"] == 9.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_series_from_post.py

@@ -93,7 +93,9 @@ async def test_add_post_stages_pending(db):
     assert grp["post"]["id"] == post.id
     assert grp["post"]["title"] == "Story pages 9-11"
     assert [pg["image_id"] for pg in grp["pages"]] == imgs
-    assert [pg["stated_page"] for pg in grp["pages"]] == [9, 10, 11]
+    # add_post stamps the post's parsed START (9) on every staged page; the
+    # group surfaces it as start_page (the default for "Place from page N").
+    assert grp["start_page"] == 9
 
 
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