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(),

src/fabledassistant/services/tz.py

@@ -13,11 +13,13 @@ from zoneinfo import ZoneInfo, ZoneInfoNotFoundError
 
 from fabledassistant.services.settings import get_setting
 
-# Briefing day boundary — kept in sync with the compilation slot in
-# ``briefing_scheduler.SLOTS``. The briefing day flips at this local hour
-# (not midnight) so the 00:00–04:00 local window still shows yesterday's
-# briefing until the 4am compilation generates the new one.
-BRIEFING_DAY_START_HOUR = 4
+# Day-rollover boundary for journal/day-anchored views. The "day" flips at
+# this local hour (not midnight) so the 00:00–04:00 local window still
+# shows yesterday's content until the user "starts" the new day.
+DAY_ROLLOVER_HOUR = 4
+
+# Backwards-compat alias for code paths still referencing the old name.
+BRIEFING_DAY_START_HOUR = DAY_ROLLOVER_HOUR
 
 
 async def get_user_tz(user_id: int) -> ZoneInfo:
@@ -35,13 +37,16 @@ async def user_today(user_id: int) -> date:
     return datetime.now(tz).date()
 
 
-async def user_briefing_date(user_id: int) -> date:
-    """Return the current "briefing day" in the user's local timezone.
+async def user_day_date(user_id: int) -> date:
+    """Return the current "day" in the user's local timezone.
 
-    The briefing day flips at ``BRIEFING_DAY_START_HOUR`` (4am local),
-    aligned with the compilation slot that generates the day's briefing.
-    Between 00:00 and 04:00 local this still returns *yesterday*, so the
-    UI keeps showing the in-progress briefing until the new one is built.
+    The day flips at ``DAY_ROLLOVER_HOUR`` (4am local) rather than midnight,
+    so the 00:00–04:00 local window still returns *yesterday* — the user's
+    journal stays anchored to yesterday until they cross the rollover.
     """
     tz = await get_user_tz(user_id)
-    return (datetime.now(tz) - timedelta(hours=BRIEFING_DAY_START_HOUR)).date()
+    return (datetime.now(tz) - timedelta(hours=DAY_ROLLOVER_HOUR)).date()
+
+
+# Backwards-compat alias used by briefing-only modules being torn down in Stage B.
+user_briefing_date = user_day_date