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/add_edges.py

@@ -37,7 +37,6 @@ async def add_chunk_statement_edges(chunks: List[Chunk], connector: Neo4jConnect
                     "apply_id": getattr(stmt, 'apply_id', None),
                     "run_id": getattr(stmt, 'run_id', None) or getattr(chunk, 'run_id', None),
                     "created_at": getattr(stmt, 'created_at', None),
-                    "expired_at": getattr(stmt, 'expired_at', None),
                     # "created_at": getattr(statement, 'created_at', None),
                     # "expired_at": None  # Set to None or appropriate default
                 }
@@ -87,7 +86,6 @@ async def add_memory_summary_statement_edges(summaries: List[MemorySummaryNode],
                     "end_user_id": s.end_user_id,
                     "run_id": s.run_id,
                     "created_at": s.created_at.isoformat() if s.created_at else None,
-                    "expired_at": s.expired_at.isoformat() if s.expired_at else None,
                 })
 
         if not edges: