feat(briefing): wire pre-processing pipeline; run_compilation returns (text, metadata)

- Task change detection via snapshot diff
- RSS scoring/filtering via briefing_preferences
- Weather card via parse_weather_card_data (staleness-gated)
- News card markdown format with ordering constraint
- Metadata stored on Message record via post_message()

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/fabledassistant/services/briefing_scheduler.py

@@ -153,9 +153,9 @@ async def _run_slot_for_user(user_id: int, slot: str) -> None:
 
         await _run_profile_closeout(user_id, model)
         conv = await get_or_create_today_conversation(user_id, model)
-        text = await run_compilation(user_id, slot, model)
+        text, metadata = await run_compilation(user_id, slot, model)
         if text:
-            await post_message(conv.id, "assistant", text)
+            await post_message(conv.id, "assistant", text, metadata=metadata)
 
     else:
         conv = await get_or_create_today_conversation(user_id, model)