fix(chat): audit fixes — retention, rag_project_id, cleanup scheduler, tool rounds

- cleanup_old_conversations now excludes briefing conversations (was
  silently deleting briefing history after the retention window)
- list_conversations response now includes rag_project_id, matching the
  shape returned by the single-conversation GET endpoint
- create_conversation_from_article: removed duplicate async_session import
  (_session2 was a copy of the same import); consolidated into one
- MAX_TOOL_ROUNDS fixed from 5→6 to match the actual range(6) loop;
  loop updated to range(MAX_TOOL_ROUNDS) so the constant is accurate
- Chat retention cleanup moved from per-request (every GET /conversations)
  to a daily scheduled job in event_scheduler.py; route no longer runs
  a DB write on every read

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

src/fabledassistant/services/chat.py

@@ -81,6 +81,7 @@ async def list_conversations(
                 "model": conv.model,
                 "conversation_type": conv.conversation_type,
                 "briefing_date": conv.briefing_date.isoformat() if conv.briefing_date else None,
+                "rag_project_id": conv.rag_project_id,
                 "message_count": row[1],
                 "created_at": conv.created_at.isoformat(),
                 "updated_at": conv.updated_at.isoformat(),
@@ -131,8 +132,9 @@ async def cleanup_old_conversations(user_id: int, days: int) -> int:
             .where(
                 Conversation.user_id == user_id,
                 Conversation.updated_at < cutoff,
-                Conversation.conversation_type != "mcp",   # preserve MCP audit trail
-                Conversation.conversation_type != "voice",  # voice convs managed separately
+                Conversation.conversation_type != "mcp",      # preserve MCP audit trail
+                Conversation.conversation_type != "voice",    # voice convs managed separately
+                Conversation.conversation_type != "briefing", # briefing history managed by briefing system
             )
             .returning(Conversation.id)
         )