Rollback session when advisory lock not acquired

Prevents PgBouncer connection pool from inheriting dirty
transaction state when the non-syncing worker returns.

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

blog/bp/blog/routes.py

@@ -55,7 +55,8 @@ def register(url_prefix, title):
       async with get_session() as s:
         got_lock = await s.scalar(text("SELECT pg_try_advisory_lock(900001)"))
         if not got_lock:
-            return  # another worker is syncing
+            await s.rollback()  # clean up before returning connection to pool
+            return
         try:
             await sync_all_content_from_ghost(s)
             await s.commit()