Fix no_autoflush: use manual toggle for async session

AsyncSession.no_autoflush is a sync context manager, can't use
with 'async with'. Toggle autoflush manually instead.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

blog/bp/blog/ghost/ghost_sync.py

@@ -185,9 +185,11 @@ async def _upsert_post(sess: AsyncSession, gp: Dict[str, Any], author_map: Dict[
                 obj.user_id = user_id
                 await sess.flush()
 
-    # Rebuild post_authors + post_tags inside no_autoflush to prevent
-    # premature INSERT from query-invoked autoflush.
-    async with sess.no_autoflush:
+    # Rebuild post_authors + post_tags with synchronize_session to keep
+    # identity map consistent and prevent autoflush IntegrityError.
+    old_autoflush = sess.autoflush
+    sess.autoflush = False
+    try:
         # Delete + re-add post_authors (dedup for Ghost duplicate authors)
         await sess.execute(
             delete(PostAuthor).where(PostAuthor.post_id == obj.id),
@@ -212,7 +214,9 @@ async def _upsert_post(sess: AsyncSession, gp: Dict[str, Any], author_map: Dict[
                 seen_tags.add(tt.id)
                 sess.add(PostTag(post_id=obj.id, tag_id=tt.id, sort_order=idx))
 
-    await sess.flush()
+        await sess.flush()
+    finally:
+        sess.autoflush = old_autoflush
 
     # Auto-create PageConfig for pages
     if obj.is_page: