chore(infra): add gRPC layer, update protobufs, remove immediate module

- Add gRPC service modules: auth, channel, channel settings, member,
  permission
- Update protobuf definitions and generated code
- Remove immediate/ real-time module (superseded by IM service)
- Update etcd discovery and registration
- Update cache, error, config, and build infrastructure
- Add ADR documentation
- Update OpenAPI spec

docs/adr/004-etcd-for-discovery.md

@@ -0,0 +1,46 @@
+# ADR-004: 使用 etcd 进行服务发现 / etcd for Service Discovery
+
+## 状态 / Status
+
+**Accepted**
+
+**日期 / Date**: 2024-01-01
+
+## 背景 / Context
+
+平台依赖多个外部 gRPC 微服务（Git、Email），需要动态发现机制来连接这些服务。
+
+The platform depends on multiple external gRPC microservices (Git, Email) and needs dynamic discovery to connect to them.
+
+## 决策 / Decision
+
+使用 **etcd** 进行服务发现和注册。
+
+Use **etcd** for service discovery and registration.
+
+## 考虑的方案 / Considered Options
+
+1. **etcd** — Kubernetes 生态标准，强一致性
+2. **Consul** — 功能丰富但较重
+3. **ZooKeeper** — 经典但运维复杂
+4. **静态配置** — 简单但不灵活
+
+## 后果 / Consequences
+
+### 正面 / Positive
+
+- 与 Kubernetes 生态一致 / Aligned with Kubernetes ecosystem
+- 强一致性保证 / Strong consistency guarantees
+- 租约机制支持健康检查 / Lease mechanism supports health checks
+- Watch 机制支持实时更新 / Watch mechanism supports real-time updates
+
+### 负面 / Negative
+
+- 额外的基础设施依赖 / Additional infrastructure dependency
+- 运维复杂度增加 / Increased operational complexity
+
+## 实现细节 / Implementation Details
+
+- **注册**: `register.rs` — 自注册 + 租约保活
+- **发现**: `discovery.rs` — Watch 动态连接
+- **客户端**: tonic/prost gRPC 客户端