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/models/conversation.py

@@ -18,11 +18,11 @@ class Conversation(Base, TimestampMixin):
     )
     title: Mapped[str] = mapped_column(Text, default="")
     model: Mapped[str] = mapped_column(Text, default="")
-    # 'chat' (default) or 'briefing'. Briefing conversations are hidden from
-    # the main chat view and managed by the briefing pipeline.
+    # 'chat' (default) or 'journal'. Journal conversations are day-anchored
+    # and rendered by the /journal view; they are hidden from the main chat list.
     conversation_type: Mapped[str] = mapped_column(Text, default="chat", server_default="chat")
-    # For briefing conversations only: the calendar date this briefing covers.
-    briefing_date: Mapped[datetime.date | None] = mapped_column(Date, nullable=True)
+    # For journal conversations only: the calendar date this conversation covers.
+    day_date: Mapped[datetime.date | None] = mapped_column(Date, nullable=True)
     # NULL = orphan notes only; -1 = all notes; positive int = specific project
     rag_project_id: Mapped[int | None] = mapped_column(Integer, nullable=True, default=None)
 
@@ -48,7 +48,7 @@ class Conversation(Base, TimestampMixin):
             "title": self.title,
             "model": self.model,
             "conversation_type": self.conversation_type,
-            "briefing_date": self.briefing_date.isoformat() if self.briefing_date else None,
+            "day_date": self.day_date.isoformat() if self.day_date else None,
             "rag_project_id": self.rag_project_id,
             "message_count": msg_count,
             "created_at": self.created_at.isoformat(),