feat(provision): 弹性节点基建 Terraform + 一键更换 (tsk_6u0FxmbC7Yeq)

IaC 面 (infra/) 与控制面 (server/internal/provision) 双产出，落地 doc/04 §4
「节点是牲口」弹性拓扑与 make-before-break 一键更换。

server/internal/provision:
- CloudAdapter 适配层 + Registry（首发 vultr 消耗品池 / hetzner 精品池各一）；
  厂商凭证仅从 PROVISION_<VENDOR>_* env 注入，不入库不入 git。
- ProvisionService：CreateNode(幂等键重放不重复开机)、DestroyNode(幂等)、
  RotateIP(换 IP 不换机 + version bump)、ListProviders。
- Replace 一键更换：先建后拆，新机 up 先于旧机 draining（容量不下降），
  replacement_uuid 幂等键 + replacements 表分步记录，崩溃可续跑不重复。
- RotatePool：池内滚动轮换，并发度 1–2。
- cmd/nodectl CLI：create/destroy/rotate-ip/replace/rotate-pool/providers。
- 单测（mock 厂商 API + 内存 Store）：幂等重放、make-before-break 时序断言、
  开机失败/探活超时→destroyed+失败计数+告警钩子、崩溃续跑、RotatePool。

infra/:
- terraform/：探针机 + 控制面基线模块化（probe / control-plane）+ README，
  低频基线进 state，节点不进 Terraform。
- cloud-init/node.yaml.tmpl：节点引导模板（注入一次性 bootstrap token，task #5）。
- identity-isolation.md：身份隔离登记表（doc/06 §2 红线，无任何凭证）。

migrations/000008：nodes 增 provider_instance_id/elastic_ip_id、node_events
增 ip_rotated、provision_idempotency / replacements 表（附加式，不动现网）。

红线：仅面向新厂商池，绝不纳管现网生产 EC2(deploy/ marzban)。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

infra/terraform/README.md

@@ -0,0 +1,67 @@
+# `infra/terraform` — baseline infrastructure as code
+
+Terraform here manages **only low-frequency baseline resources**:
+
+- **probe machines** — overseas reference dial-test points (doc/04 §1, §4.2).
+- **control-plane environment** — the API + MySQL + Redis host (doc/04 §5.2),
+  off by default because it changes rarely.
+
+## What is deliberately NOT here
+
+**Data-plane nodes are not in Terraform.** They are disposable cattle, created
+and destroyed minute-by-minute through vendor APIs by the provision service
+(`server/internal/provision`, doc/04 §4). Putting them in Terraform state would
+fight the "nodes are cattle, not pets" model and serialise every open/退机
+behind a state lock. Division of labour:
+
+| concern | owner | frequency | in TF state |
+|---------|-------|-----------|-------------|
+| probe machines | Terraform (this dir) | low | ✅ |
+| control-plane env | Terraform (this dir) | one-off | ✅ |
+| data-plane nodes (entry/relay/exit) | provision service + vendor API | minutes | ❌ |
+
+## Identity isolation (RED LINE, doc/06 §2)
+
+Every vendor/account/domain used here MUST be an **independent identity** — its
+own account, email, and crypto payment, with **zero cross-linkage** to other
+assets. This Terraform code and the data-plane vendor accounts use **separate**
+credentials:
+
+- `var.hcloud_token` here is the **management/probe** account token — *not* a
+  data-plane vendor token.
+- Data-plane vendor credentials live only in `PROVISION_<VENDOR>_*` env secrets
+  consumed by the provision service, never in Terraform, never in the DB.
+
+The full isolation register (which account/email/payment maps to which asset) is
+maintained in [`../identity-isolation.md`](../identity-isolation.md).
+
+## Usage
+
+```bash
+cd infra/terraform
+cp terraform.tfvars.example terraform.tfvars   # gitignored; fill in
+export TF_VAR_hcloud_token=...                 # inject secret, never commit
+
+terraform init
+terraform fmt -check
+terraform validate
+terraform plan
+terraform apply
+```
+
+CI/management operations run on a **dedicated channel** (doc/06 §2), not from an
+operator's everyday machine.
+
+## Layout
+
+```
+terraform/
+  versions.tf            provider + backend pins
+  variables.tf           root inputs
+  main.tf                wires probe[] + control-plane modules
+  outputs.tf             probe IPs, control-plane IP
+  terraform.tfvars.example
+  modules/
+    probe/               one overseas reference probe point
+    control-plane/       API+MySQL+Redis host (prevent_destroy)
+```