refactor(application): filter_rules.py 更名 rules.py 并合并 RuleHelper

规则域收敛为单一事实来源 app/application/rules.py：
- filter_rules.py 更名为 rules.py（内置规则集 + RuleParser）
- filter.py 仅含 66 行 RuleHelper（用户规则组配置访问），并入 rules.py
- 同步更新 5 处引用方导入、nettest stub 目标与架构文档
- 兼容映射 app.helper.rule 指向新路径，legacy 导入仍可用

app/api/endpoints/system.py

@@ -41,7 +41,7 @@ from app.adapters.external.market import (
 )
 from app.application.messaging.message import MessageHelper
 from app.runtime.progress import ProgressHelper
-from app.application.filter import RuleHelper
+from app.application.rules import RuleHelper
 from app.adapters.external.server import MoviePilotServerHelper
 from app.runtime.state import SystemHelper
 from app.runtime.log import logger