fix: align sender paths with plan, remove QQ-as-universal fallback

- Remove get_bot_account("qq") fallback from all 4 sender paths
  (plan L108/L208/L449: unknown platform = no account, never substitute QQ)
- Sender paths now error immediately if platform bot account is not configured
- Add detailed comments on filter_bot legacy fallback explaining why
  global user_id match is needed (plan contingency L528 insufficient for
  platform-tagged legacy rows like telegram+qq_account)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/services/send_service.py

@@ -85,9 +85,7 @@ async def _send_to_target(
             additional_config["selected_expressions"] = selected_expressions
         bot_user_id = get_bot_account(target_stream.platform)
         if not bot_user_id:
-            bot_user_id = get_bot_account("qq")
-        if not bot_user_id:
-            logger.error(f"[SendService] 平台 {target_stream.platform} 无可用机器人账号，无法发送消息")
+            logger.error(f"[SendService] 平台 {target_stream.platform} 未配置机器人账号，无法发送消息")
             return False
 
         maim_message = MessageBase(