fix: resolve E2E audit findings and add Phase C frontend pages

- Fix audit_log handler multi-tenant bug: use Extension<TenantContext>
  instead of hardcoded default_tenant_id
- Fix sendMessage route mismatch: frontend /messages/send → /messages
- Add POST /users/{id}/roles backend route for role assignment
- Add task.completed event payload: started_by + instance_id for
  notification delivery
- Add audit log viewer frontend page (AuditLogViewer.tsx)
- Add language management frontend page (LanguageManager.tsx)
- Add api/auditLogs.ts and api/languages.ts modules

crates/erp-auth/src/service/user_service.rs

@@ -277,6 +277,70 @@ impl UserService {
         Ok(())
     }
 
+    /// Replace all role assignments for a user within a tenant.
+    pub async fn assign_roles(
+        user_id: Uuid,
+        tenant_id: Uuid,
+        operator_id: Uuid,
+        role_ids: &[Uuid],
+        db: &sea_orm::DatabaseConnection,
+    ) -> AuthResult<Vec<RoleResp>> {
+        // 验证用户存在
+        let _user = user::Entity::find_by_id(user_id)
+            .one(db)
+            .await
+            .map_err(|e| AuthError::Validation(e.to_string()))?
+            .filter(|u| u.tenant_id == tenant_id && u.deleted_at.is_none())
+            .ok_or_else(|| AuthError::Validation("用户不存在".to_string()))?;
+
+        // 验证所有角色存在且属于当前租户
+        if !role_ids.is_empty() {
+            let found = role::Entity::find()
+                .filter(role::Column::Id.is_in(role_ids.iter().copied()))
+                .filter(role::Column::TenantId.eq(tenant_id))
+                .all(db)
+                .await
+                .map_err(|e| AuthError::Validation(e.to_string()))?;
+            if found.len() != role_ids.len() {
+                return Err(AuthError::Validation("部分角色不存在或不属于当前租户".to_string()));
+            }
+        }
+
+        // 删除旧的角色分配
+        user_role::Entity::delete_many()
+            .filter(user_role::Column::UserId.eq(user_id))
+            .filter(user_role::Column::TenantId.eq(tenant_id))
+            .exec(db)
+            .await
+            .map_err(|e| AuthError::Validation(e.to_string()))?;
+
+        // 创建新的角色分配
+        let now = chrono::Utc::now();
+        for &role_id in role_ids {
+            let assignment = user_role::ActiveModel {
+                user_id: Set(user_id),
+                role_id: Set(role_id),
+                tenant_id: Set(tenant_id),
+                created_at: Set(now),
+                updated_at: Set(now),
+                created_by: Set(operator_id),
+                updated_by: Set(operator_id),
+                deleted_at: Set(None),
+                version: Set(1),
+            };
+            assignment.insert(db).await.map_err(|e| AuthError::Validation(e.to_string()))?;
+        }
+
+        audit_service::record(
+            AuditLog::new(tenant_id, Some(operator_id), "user.assign_roles", "user")
+                .with_resource_id(user_id),
+            db,
+        )
+        .await;
+
+        Self::fetch_user_role_resps(user_id, tenant_id, db).await
+    }
+
     /// Fetch RoleResp DTOs for a given user within a tenant.
     async fn fetch_user_role_resps(
         user_id: Uuid,