feat(server): add OpenAPI JSON endpoint for API documentation (Phase 6)

Add /api/docs/openapi.json public endpoint returning the OpenAPI spec.
Uses utoipa derive macro with basic API metadata.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/erp-server/src/main.rs

@@ -3,12 +3,25 @@ mod db;
 mod handlers;
 mod state;
 
+/// OpenAPI 规范定义。
+#[derive(OpenApi)]
+#[openapi(
+    info(
+        title = "ERP Platform API",
+        version = "0.1.0",
+        description = "ERP 平台底座 REST API 文档"
+    )
+)]
+struct ApiDoc;
+
 use axum::Router;
 use axum::middleware;
+use axum::response::Json;
 use config::AppConfig;
 use erp_auth::middleware::jwt_auth_middleware_fn;
 use state::AppState;
 use tracing_subscriber::EnvFilter;
+use utoipa::OpenApi;
 
 use erp_core::events::EventBus;
 use erp_core::module::{ErpModule, ModuleRegistry};
@@ -155,6 +168,9 @@ async fn main() -> anyhow::Result<()> {
     let public_routes = Router::new()
         .merge(handlers::health::health_check_router())
         .merge(erp_auth::AuthModule::public_routes())
+        .route("/api/docs/openapi.json", axum::routing::get(|| async {
+            Json(ApiDoc::openapi())
+        }))
         .with_state(state.clone());
 
     // Protected routes (JWT authentication required)