feat: system knowledge base management with admin/user role separation

- Auto-create system KBs from data/knowledge_base/ subdirectories on startup
- Upgrade existing user KBs to system KBs when matching directory found
- Admin can upload to system KBs (saved to knowledge_base dir)
- Regular users see only their own KBs on knowledge page
- Fix API permission checks for system KBs (list, detail, documents, update, delete)
- Add is_superuser to UserResponse and frontend User type
- Fix sidebar session menu visibility for long titles

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/main.py

@@ -128,6 +128,17 @@ async def startup_event():
     except Exception as e:
         print(f"启动知识库服务失败: {e}", file=sys.stderr)
 
+    # 确保系统知识库与目录同步
+    try:
+        from src.core.database import get_db
+        from src.services.knowledge_base_service import KnowledgeBaseService
+        db = next(get_db())
+        kb_service = KnowledgeBaseService(db)
+        kb_service.ensure_system_knowledge_bases()
+        db.close()
+    except Exception as e:
+        print(f"同步系统知识库失败: {e}", file=sys.stderr)
+
     # 初始化论坛分类
     try:
         seed_forum_categories()