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

tests/test_system_nettest.py

@@ -51,7 +51,7 @@ _STUB_MODULES = dict([
     _stub("app.application.mediaserver", MediaServerHelper=_Dummy),
     _stub("app.application.messaging.message", MessageHelper=_Dummy),
     _stub("app.runtime.progress", ProgressHelper=_Dummy),
-    _stub("app.application.filter", RuleHelper=_Dummy),
+    _stub("app.application.rules", RuleHelper=_Dummy),
     _stub("app.adapters.external.server", MoviePilotServerHelper=_Dummy),
     _stub("app.runtime.state", SystemHelper=_Dummy),
     _stub("app.application.image", ImageHelper=_Dummy),