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-server/src/handlers/openapi.rs

@@ -0,0 +1,25 @@
+use axum::response::{IntoResponse, Json, Response};
+use utoipa::OpenApi;
+
+use crate::{ApiDoc, AuthApiDoc, ConfigApiDoc, MessageApiDoc, WorkflowApiDoc};
+
+/// GET /docs/openapi.json
+///
+/// 返回 OpenAPI 3.0 规范 JSON 文档，合并所有模块的路径和 schema。
+/// 仅在 debug 模式下可用，生产构建返回 404。
+pub async fn openapi_spec() -> Response {
+    #[cfg(debug_assertions)]
+    {
+        let mut spec = ApiDoc::openapi();
+        spec.merge(AuthApiDoc::openapi());
+        spec.merge(ConfigApiDoc::openapi());
+        spec.merge(WorkflowApiDoc::openapi());
+        spec.merge(MessageApiDoc::openapi());
+        Json(serde_json::to_value(spec).unwrap_or_default()).into_response()
+    }
+
+    #[cfg(not(debug_assertions))]
+    {
+        (axum::http::StatusCode::NOT_FOUND, "Not Found").into_response()
+    }
+}