refactor(memory): remove expired_at field and add dialog_at timestamp

Remove the deprecated expired_at field from all graph models, Neo4j
Cypher queries, repositories, and pipeline code. Replace with dialog_at
on StatementNode to track the original dialog timestamp.

- Strip expired_at from DialogueNode, ChunkNode, StatementNode,
  ExtractedEntityNode, edges, and all Cypher queries
- Add dialog_at to MessageItem schema and propagate through extraction
  and graph build steps
- Extract emotion/metadata async submission from WritePipeline into
  a generic _submit_celery_task helper
- Add post_store_dedup_and_alias_merge Celery task for async alias
  merging and second-layer dedup after Neo4j write
- Switch pytest async backend from anyio to asyncio_mode=auto

api/app/repositories/neo4j/entity_repository.py

@@ -48,8 +48,6 @@ class EntityRepository(BaseNeo4jRepository[ExtractedEntityNode]):
         # 处理datetime字段
         if isinstance(n.get('created_at'), str):
             n['created_at'] = datetime.fromisoformat(n['created_at'])
-        if n.get('expired_at') and isinstance(n.get('expired_at'), str):
-            n['expired_at'] = datetime.fromisoformat(n['expired_at'])
         
         # 确保aliases字段存在且为列表
         if 'aliases' not in n or n['aliases'] is None: