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- Remove immediate/ real-time module (superseded by IM service)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/005-error-handling-strategy.md

@@ -0,0 +1,68 @@
+# ADR-005: 统一错误处理策略 / Unified Error Handling Strategy
+
+## 状态 / Status
+
+**Accepted**
+
+**日期 / Date**: 2024-01-01
+
+## 背景 / Context
+
+平台需要统一的错误处理机制，确保错误信息对用户友好，同时保留足够的调试信息。
+
+The platform needs a unified error handling mechanism that provides user-friendly error messages while retaining sufficient debugging information.
+
+## 决策 / Decision
+
+使用 **AppError 枚举 + AppResult 类型别名** 作为统一错误处理策略。
+
+Use **AppError enum + AppResult type alias** as the unified error handling strategy.
+
+## 考虑的方案 / Considered Options
+
+1. **自定义枚举 (AppError)** — 类型安全、可扩展
+2. **anyhow** — 简单但类型信息丢失
+3. **thiserror + 手动实现** — 灵活但工作量大
+
+## 后果 / Consequences
+
+### 正面 / Positive
+
+- 类型安全的错误处理 / Type-safe error handling
+- 统一的错误响应格式 / Unified error response format
+- 便于错误分类和监控 / Easy error classification and monitoring
+- 与 actix-web 集成良好 / Good integration with actix-web
+
+### 负面 / Negative
+
+- 需要维护错误枚举 / Need to maintain error enum
+- 新增错误类型需要更新枚举 / New error types require enum updates
+
+## 实现细节 / Implementation Details
+
+```rust
+// AppError 定义
+#[derive(Debug, thiserror::Error)]
+pub enum AppError {
+    #[error("User not found")]
+    UserNotFound,
+    #[error("Invalid password")]
+    InvalidPassword,
+    #[error("Database error: {0}")]
+    Database(#[from] sqlx::Error),
+    // ...
+}
+
+// AppResult 类型别名
+pub type AppResult<T> = Result<T, AppError>;
+```
+
+## 错误码映射 / Error Code Mapping
+
+| Postgres Code | 含义 | HTTP Status |
+|---|---|---|
+| 23505 | 唯一约束违反 | 409 Conflict |
+| 23503 | 外键约束违反 | 400 Bad Request |
+| 23514 | 检查约束违反 | 400 Bad Request |
+| 23502 | 非空约束违反 | 400 Bad Request |
+| 23P01 | 排他约束违反 | 409 Conflict |