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/__init__.py

@@ -43,3 +43,11 @@ from fabledassistant.models.weather_cache import WeatherCache  # noqa: E402, F40
 from fabledassistant.models.api_key import ApiKey  # noqa: E402, F401
 from fabledassistant.models.user_profile import UserProfile  # noqa: E402, F401
 from fabledassistant.models.rss_item_embedding import RssItemEmbedding  # noqa: E402, F401
+from fabledassistant.models.moment import (  # noqa: E402, F401
+    Moment,
+    MomentEmbedding,
+    moment_people,
+    moment_places,
+    moment_tasks,
+    moment_notes,
+)