Fix _upsert_post savepoint: add() inside begin_nested()

begin_nested() auto-flushes on entry which triggers the INSERT before
the savepoint is active. Move sess.add() inside the savepoint block
and split into update vs insert paths.

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

bp/blog/ghost/ghost_sync.py

@@ -214,23 +214,26 @@ async def _upsert_post(sess: AsyncSession, gp: Dict[str, Any], author_map: Dict[
 
     res = await sess.execute(select(Post).where(Post.ghost_id == gp["id"]))
     obj = res.scalar_one_or_none()
-    if obj is None:
-        obj = Post(ghost_id=gp["id"])  # type: ignore[call-arg]
-        sess.add(obj)
 
-    _apply_ghost_fields(obj, gp, author_map, tag_map)
-
-    try:
-        async with sess.begin_nested():
-            await sess.flush()
-    except IntegrityError:
-        # Race condition: another request inserted this ghost_id concurrently.
-        # Expunge the failed object, re-select the existing row, and update it.
-        sess.expunge(obj)
-        res = await sess.execute(select(Post).where(Post.ghost_id == gp["id"]))
-        obj = res.scalar_one()
+    if obj is not None:
+        # Row exists — just update
         _apply_ghost_fields(obj, gp, author_map, tag_map)
         await sess.flush()
+    else:
+        # Row doesn't exist — try to insert within a savepoint
+        obj = Post(ghost_id=gp["id"])  # type: ignore[call-arg]
+        try:
+            async with sess.begin_nested():
+                sess.add(obj)
+                _apply_ghost_fields(obj, gp, author_map, tag_map)
+                await sess.flush()
+        except IntegrityError:
+            # Race condition: another request inserted this ghost_id.
+            # Savepoint rolled back; re-select and update.
+            res = await sess.execute(select(Post).where(Post.ghost_id == gp["id"]))
+            obj = res.scalar_one()
+            _apply_ghost_fields(obj, gp, author_map, tag_map)
+            await sess.flush()
 
     # Backfill user_id from primary author email if not already set
     if obj.user_id is None and obj.primary_author_id is not None: