feat: inject briefing article content for deep article Q&A

Add _build_briefing_article_context() helper to llm.py that reads
rss_item_ids from briefing message metadata and injects article content
into the system prompt. Pass conv_id through build_context() and
generation_task.py.

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

src/fabledassistant/services/generation_task.py

@@ -185,6 +185,7 @@ async def run_generation(
         rag_project_id=rag_project_id,
         workspace_project_id=workspace_project_id,
         user_timezone=user_timezone,
+        conv_id=conv_id,
     ))
 
     messages, context_meta = await context_task