feat(models): Moment + MomentEmbedding + junctions; rename Conversation.briefing_date → day_date

Also rename services/tz.user_briefing_date → user_day_date with a backwards
compat alias (briefing modules using the old name will be deleted in the
upcoming briefing tear-down stage). Update services/chat.py to_dict to use
day_date.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/fabledassistant/services/chat.py

@@ -80,7 +80,7 @@ async def list_conversations(
                 "title": conv.title,
                 "model": conv.model,
                 "conversation_type": conv.conversation_type,
-                "briefing_date": conv.briefing_date.isoformat() if conv.briefing_date else None,
+                "day_date": conv.day_date.isoformat() if conv.day_date else None,
                 "rag_project_id": conv.rag_project_id,
                 "message_count": row[1],
                 "created_at": conv.created_at.isoformat(),