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/schemas/memory_agent_schema.py

@@ -1,6 +1,7 @@
 from abc import ABC
 from enum import Enum
 from typing import Any, Optional
+from pydantic import Field
 
 from pydantic import BaseModel
 
@@ -21,6 +22,10 @@ class MessageItem(BaseModel):
     """单条消息结构"""
     role: str
     content: str
+    dialog_at: Optional[str] = Field(
+        None,
+        description="该条消息发生的绝对时间（ISO 8601 格式），不传则使用服务端当前时间",
+    )
     files: Optional[list[dict]] = None
     file_content: Optional[list[Any]] = None