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

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

backend/app/services/tag_service.py

@@ -307,6 +307,7 @@ class TagService:
         await self._repoint_fandom_children(
             source_id, target_id, source_kind
         )
+        await self._repoint_series_pages(source_id, target_id)
 
         alias_created = False
         if keep_as_alias:
@@ -400,6 +401,17 @@ class TagService:
             .values(canonical_tag_id=tgt)
         )
 
+    async def _repoint_series_pages(self, src: int, tgt: int) -> None:
+        from ..models.series_page import SeriesPage
+
+        # image_id is UNIQUE across series_page and src != tgt, so an
+        # image in src's series cannot already be in tgt's — no collision.
+        await self.session.execute(
+            update(SeriesPage)
+            .where(SeriesPage.series_tag_id == src)
+            .values(series_tag_id=tgt)
+        )
+
     async def _repoint_fandom_children(
         self, src: int, tgt: int, src_kind: TagKind
     ) -> None: