refactor(notifications): deduplicate AlertMarkerStore + add pluggable channel Protocols

[Pigbibi]

Summary

通知模块两项重构：消除重复代码 + 添加可插拔渠道 Protocol。

1. 消除 4x 重复的 AlertMarkerStore（-164 行）

新增 alert_marker.py，抽取共享的 CloudAlertMarkerStore 基类：

• has_alert() / record_alert() / _local_path() / _cloud_uri() 统一实现
• _clean_relative_key 和 _parse_cloud_uri 统一到基类（同时支持 gs://, s3://, az://）
• 4 个渠道各自的 StrategyPlugin*AlertMarkerStore 从 ~55 行缩减为 3 行子类

改动前（4 个文件各 55 行重复）：

class StrategyPluginTelegramAlertMarkerStore:
    def has_alert(...): ...
    def record_alert(...): ...
    def _local_path(...): ...
    def _cloud_uri(...): ...
    def _object_store(...): ...

改动后：

class StrategyPluginTelegramAlertMarkerStore(CloudAlertMarkerStore):
    namespace: str = "strategy_plugin_telegram_alerts"
    schema_version: str = "strategy_plugin_telegram_alert_marker.v1"

2. 可插拔通知渠道 Protocol

新增 channel.py，定义 4 个 Protocol + 5 个默认实现：

Protocol	默认实现	可替换为
SmsChannel	TwilioSmsChannel	阿里云短信、AWS SNS
PushChannel	PushoverChannel + NtfyChannel	企业微信、Slack
EmailChannel	SmtpEmailChannel	SendGrid、AWS SES
ChatChannel	TelegramChatChannel	Discord、WhatsApp

替换示例：

from quant_platform_kit.notifications import SmsChannel

class AliyunSmsChannel:
    def send_sms(self, recipient, body, *, sender=None):
        ...  # call Aliyun API
        return True

publish_strategy_plugin_sms_alerts(
    signals,
    sms_settings=settings,
    send_notification=AliyunSmsChannel().send_sms,
)

Testing

• 373 passed, 0 failed, 1 skipped (pre-existing email.py/stdlib collision)

Breaking Changes

None. All existing public APIs preserved.