Fix Ghost sync race: advisory lock for multi-worker startup

Two Hypercorn workers both run sync_all_content_from_ghost on startup,
racing on PostAuthor/PostTag rows. Use pg_try_advisory_lock so only
one worker runs the sync.

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

blog/bp/blog/routes.py

@@ -48,10 +48,20 @@ def register(url_prefix, title):
     @blogs_bp.before_app_serving
     async def init():
       from .ghost.ghost_sync import sync_all_content_from_ghost
+      from sqlalchemy import text
 
+      # Advisory lock prevents multiple Hypercorn workers from
+      # running the sync concurrently (which causes PK conflicts).
       async with get_session() as s:
-        await sync_all_content_from_ghost(s)
-        await s.commit()
+        got_lock = await s.scalar(text("SELECT pg_try_advisory_lock(900001)"))
+        if not got_lock:
+            return  # another worker is syncing
+        try:
+            await sync_all_content_from_ghost(s)
+            await s.commit()
+        finally:
+            await s.execute(text("SELECT pg_advisory_unlock(900001)"))
+            await s.commit()
 
     @blogs_bp.before_request
     def route():