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/routes/briefing.py

@@ -232,6 +232,6 @@ async def manual_trigger():
 
     model = await get_setting(g.user.id, "default_model", "")
     conv = await get_or_create_today_conversation(g.user.id, model)
-    text = await run_compilation(g.user.id, slot, model)
-    msg = await post_message(conv.id, "assistant", text)
+    text, metadata = await run_compilation(g.user.id, slot, model)
+    msg = await post_message(conv.id, "assistant", text, metadata=metadata)
     return jsonify({"conversation_id": conv.id, "message_id": msg.id, "slot": slot})