feat(backend): 挂载 /v1 API + 实现 nodes/connect 端到端

- 新增 internal/dpcred 包，统一 DeriveHy2Password + DefaultFlow
  agentd 与 HTTP connect handler 共享同一实现
- 新增迁移 000011：nodes 表拆分 reality_prk 私钥 / reality_pbk 公钥
  reality_short_id；修正 handler_grpc.go 使用私钥字段
- 新增迁移 000012：connect_credentials 持久化凭证
  实现 CredentialsForNode 修复 agent 重连 resync 原先返回空的桩
- 扩展 NodeStore 接口：ListUp / EntitlementForUser /
  PersistCredential / DeleteCredential；同步 grpc_test.go mock
- 新增 httpapi/nodes.go：GET /nodes、POST /nodes/id/connect
  Hub.Push + PersistCredential + 渲染完整 sing-box client 配置 JSON
  POST /nodes/id/disconnect
- 新增 httpapi/account.go：GET /me、GET /plans、GET /notices
- 新增 httpapi/clientconfig.go：BuildClientConfig 服务端渲染
- 重写 cmd/server/main.go：手写 chi public/protected 分组
  nodes.Service/Hub 在 main 构造并共享；SMTPMailer/LogMailer

go build ./... && go vet ./... && go test ./... 全部通过

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

server/internal/nodes/grpc_test.go

@@ -80,6 +80,22 @@ func (m *mockNodeStore) UserIDByDpUUID(_ context.Context, dpUUID string) (int64,
 	return 0, false, nil
 }
 
+func (m *mockNodeStore) ListUp(_ context.Context) ([]*nodes.NodeRow, error) {
+	return []*nodes.NodeRow{{ID: 1, UUID: m.nodeUUID, Status: "up"}}, nil
+}
+
+func (m *mockNodeStore) EntitlementForUser(_ context.Context, _ int64) (*nodes.Entitlement, error) {
+	return nil, nil
+}
+
+func (m *mockNodeStore) PersistCredential(_ context.Context, _ int64, _ *agentv1.Credential, _ time.Time) error {
+	return nil
+}
+
+func (m *mockNodeStore) DeleteCredential(_ context.Context, _ int64, _ string) error {
+	return nil
+}
+
 func (m *mockNodeStore) AccumulateUsage(_ context.Context, userID int64, date time.Time,
 	bytesUp, bytesDown, minutes int64,
 ) error {