feat: initialize Nuanji (Warm Notes) project

- Base platform from base.git (ERP base: auth, core, config, message, workflow, plugin)
- Created erp-diary module skeleton (lib.rs, dto.rs, error.rs, event.rs, state.rs)
- Integrated erp-diary into workspace and erp-server
- Added DiaryModule registration in main.rs
- Added DiaryState FromRef in state.rs
- Diary routes mounted (empty routes, ready for implementation)
- Product design spec v1.2 preserved in docs/
- Implementation plan preserved in plans/

Cargo check: OK
Cargo test: OK (78+ base tests passing)

crates/erp-diary/src/error.rs

@@ -0,0 +1,75 @@
+// erp-diary 错误类型
+
+use axum::http::StatusCode;
+use axum::response::{IntoResponse, Response};
+use serde::Serialize;
+
+#[derive(Debug, thiserror::Error)]
+pub enum DiaryError {
+    #[error("日记未找到: {0}")]
+    NotFound(String),
+
+    #[error("版本冲突: 本地版本 {local}, 服务端版本 {server}")]
+    VersionConflict { local: i32, server: i32 },
+
+    #[error("班级码无效")]
+    InvalidClassCode,
+
+    #[error("班级码已过期")]
+    ClassCodeExpired,
+
+    #[error("班级码尝试次数过多，请 {lockout_minutes} 分钟后重试")]
+    ClassCodeLocked { lockout_minutes: u32 },
+
+    #[error("无权限执行此操作")]
+    Forbidden,
+
+    #[error("内容安全检查未通过")]
+    ContentSafetyViolation,
+
+    #[error("同步失败: {0}")]
+    SyncFailed(String),
+
+    #[error("{0}")]
+    BadRequest(String),
+
+    #[error("内部错误: {0}")]
+    Internal(String),
+}
+
+#[derive(Serialize)]
+struct ErrorBody {
+    error: String,
+    message: String,
+}
+
+impl IntoResponse for DiaryError {
+    fn into_response(self) -> Response {
+        let (status, message) = match &self {
+            DiaryError::NotFound(_) => (StatusCode::NOT_FOUND, self.to_string()),
+            DiaryError::VersionConflict { .. } => (StatusCode::CONFLICT, self.to_string()),
+            DiaryError::InvalidClassCode | DiaryError::ClassCodeExpired => {
+                (StatusCode::BAD_REQUEST, self.to_string())
+            }
+            DiaryError::ClassCodeLocked { .. } => (StatusCode::TOO_MANY_REQUESTS, self.to_string()),
+            DiaryError::Forbidden => (StatusCode::FORBIDDEN, self.to_string()),
+            DiaryError::ContentSafetyViolation => (StatusCode::BAD_REQUEST, self.to_string()),
+            DiaryError::SyncFailed(_) => (StatusCode::INTERNAL_SERVER_ERROR, self.to_string()),
+            DiaryError::BadRequest(_) => (StatusCode::BAD_REQUEST, self.to_string()),
+            DiaryError::Internal(_) => (StatusCode::INTERNAL_SERVER_ERROR, self.to_string()),
+        };
+
+        let body = ErrorBody {
+            error: format!("diary.{}", status.as_u16()),
+            message,
+        };
+
+        (status, axum::Json(body)).into_response()
+    }
+}
+
+impl From<sea_orm::DbErr> for DiaryError {
+    fn from(err: sea_orm::DbErr) -> Self {
+        DiaryError::Internal(err.to_string())
+    }
+}