feat(series): TagService.merge repoints series_page.series_tag_id

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

tests/test_tag_merge.py

@@ -410,3 +410,20 @@ async def test_alias_create_does_not_clobber_existing(db):
         )
     )
     assert cid == other.id  # pre-existing alias untouched
+
+
+@pytest.mark.asyncio
+async def test_merge_repoints_series_pages(db):
+    from backend.app.models.series_page import SeriesPage
+
+    svc = TagService(db)
+    a = await svc.find_or_create("SerA", TagKind.series)
+    b = await svc.find_or_create("SerB", TagKind.series)
+    img = await _img(db)
+    db.add(SeriesPage(series_tag_id=a.id, image_id=img, page_number=1))
+    await db.flush()
+    await svc.merge(a.id, b.id)
+    owner = await db.scalar(
+        select(SeriesPage.series_tag_id).where(SeriesPage.image_id == img)
+    )
+    assert owner == b.id