feat(multicloud): 补齐上线前的多云管理缺口（适配器/动作层/DNS/定时同步）

评估发现项目只有台账能力，缺失真正的多云管理链路。本次按缺口逐项补齐：

适配层
- 新增 aws / linode / namesilo 适配器，registry 登记；阿里云、腾讯云补
  list_domains 与 get_account 余额
- NormalizedVPS 补 expiry_date + charge_type：此前云端同步进来的 VPS 拿不到
  到期日，永远进不了续费提醒——而续费提醒正是这套台账存在的理由
- base.py 统一云端错误可读化（http_error_message / cloud_error_text）：各厂商
  把原因放的位置都不同（Linode errors[].reason、Cloudflare errors[].message、
  腾讯 Response.Error.Message、AWS XML），此前直接 str(e) 会把 httpx 的多行文本
  连同 MDN 链接弹给用户

动作层与 DNS
- 新增 /api/actions：实例启停/重启/续费，写操作全部二次确认 + 审计
- 新增 Cloudflare DNS 记录的增删改查

定时同步
- scripts/sync_all_assets.py + /api/sync/assets + vps-sync-assets.timer(08:30)
- 08:30 而非随机时刻：必须早于 09:00 的续费检查，否则提醒读到上一天的旧到期日
- sync_all_assets 修正计数：每一项子同步都失败时不再计为「成功」，否则凭证过期
  会显示「成功 1」，用户以为数据拉到了、实际一条没有

数据一致性
- AssetRead 输出 external_id（只出不进）：前端「云操作」「DNS」按钮靠它渲染，
  缺了则所有按钮永远出不来
- import_assets 落库 external_id：导出带了去重键而导入丢弃，会让迁移后首次同步
  把所有云端资产当新增再建一份
- _align_preset_providers 取代 _backfill_provider_services：库里声明但注册表未
  实现的 sdk_type 清空（声明支持却不支持，比明说不支持更糟），services 只增量
  补标签不覆盖，用户自建平台不动

前端如实展示
- cost 为 0 统一显示「未填」而非「0 USD」：0 既可能是没填、也可能是云端不返回
  价格，而汇总处又把 0 当「无费用」排除，两处口径不一致
- 总览「本年支出」增加 year_cost_missing，把合计的覆盖缺口说出来
- DNS 弹窗补 MX/SRV 优先级与备注；列表请求失败时 provider 用资产自带 slug 兜底，
  否则 Cloudflare 代理勾选框永远不出现

测试：257 -> 309 passed

app/core/seed.py

@@ -23,21 +23,27 @@ PRESET_PROVIDERS = [
     {"slug": "tencent-intl", "name": "腾讯国际", "name_en": "Tencent Cloud Intl", "category": "vps",
      "services": "vps,domain,ssl_cert,cdn,dns",
      "website": "https://intl.cloud.tencent.com", "console_url": "https://console.intl.cloud.tencent.com", "sdk_type": "tencent-intl-sdk"},
+    {"slug": "aws", "name": "AWS", "name_en": "Amazon Web Services", "category": "vps",
+     "services": "vps,domain,dns,cdn,ssl_cert",
+     "website": "https://aws.amazon.com", "console_url": "https://console.aws.amazon.com/ec2/", "sdk_type": "aws-api"},
     {"slug": "vultr", "name": "Vultr", "name_en": "Vultr", "category": "vps",
-     "services": "vps",
+     "services": "vps,domain,dns",
      "website": "https://vultr.com", "console_url": "https://my.vultr.com", "sdk_type": "vultr-api"},
     {"slug": "digitalocean", "name": "DigitalOcean", "name_en": "DigitalOcean", "category": "vps",
-     "services": "vps,domain",
+     "services": "vps,domain,dns",
      "website": "https://digitalocean.com", "console_url": "https://cloud.digitalocean.com", "sdk_type": "do-api"},
     {"slug": "linode", "name": "Linode", "name_en": "Akamai Linode", "category": "vps",
-     "services": "vps",
+     "services": "vps,domain,dns",
      "website": "https://linode.com", "console_url": "https://cloud.linode.com", "sdk_type": "linode-api"},
+    # 以下平台暂无适配器：sdk_type 必须留空。
+    # 填了不存在的 sdk_type 会让平台卡片显示「API 对接: xxx」，用户建账号点同步才撞
+    # 「暂不支持的 SDK 类型」——声明支持却不支持，比明说不支持更糟。
     {"slug": "cloudcone", "name": "CloudCone", "name_en": "CloudCone", "category": "vps",
      "services": "vps",
-     "website": "https://cloudcone.com", "console_url": "https://app.cloudcone.com", "sdk_type": "cloudcone-api"},
+     "website": "https://cloudcone.com", "console_url": "https://app.cloudcone.com", "sdk_type": None},
     {"slug": "zeabur", "name": "Zeabur", "name_en": "Zeabur", "category": "vps",
      "services": "vps,domain",
-     "website": "https://zeabur.com", "console_url": "https://dash.zeabur.com", "sdk_type": "zeabur-api"},
+     "website": "https://zeabur.com", "console_url": "https://dash.zeabur.com", "sdk_type": None},
     # ---- 域名注册商 ----
     {"slug": "namesilo", "name": "Namesilo", "name_en": "Namesilo", "category": "domain",
      "services": "domain,dns",
@@ -47,7 +53,7 @@ PRESET_PROVIDERS = [
      "website": "https://xinnet.com", "console_url": "https://www.xinnet.com", "sdk_type": None},
     {"slug": "dynadot", "name": "Dynadot", "name_en": "Dynadot", "category": "domain",
      "services": "domain,dns",
-     "website": "https://dynadot.com", "console_url": "https://www.dynadot.com/account/domains", "sdk_type": "dynadot-api"},
+     "website": "https://dynadot.com", "console_url": "https://www.dynadot.com/account/domains", "sdk_type": None},
     # ---- AI 服务商 ----
     {"slug": "openai", "name": "OpenAI", "name_en": "OpenAI", "category": "ai_agent",
      "services": "ai_agent",