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/celery_app.py

@@ -117,6 +117,12 @@ celery_app.conf.update(
         # Async emotion extraction → memory_tasks queue (IO-bound LLM calls)
         'app.tasks.extract_emotion_batch': {'queue': 'memory_tasks'},
 
+        # Post-store dedup + alias merge → memory_tasks queue
+        'app.tasks.post_store_dedup_and_alias_merge': {'queue': 'memory_tasks'},
+
+        # Async metadata extraction → memory_tasks queue
+        'app.tasks.extract_metadata_batch': {'queue': 'memory_tasks'},
+
         # Document tasks → document_tasks queue (prefork worker)
         'app.core.rag.tasks.parse_document': {'queue': 'document_tasks'},
         'app.core.rag.tasks.sync_knowledge_for_kb': {'queue': 'document_tasks'},