refactor: introduce service layer and migrate gRPC handlers to use it

Extract business logic from gRPC handlers into a dedicated service package.
Add context support throughout for cancellation and timeouts. Move models to
their own package, remove hardcoded credentials from config, and simplify
parsers to only handle HTML parsing.

service/login.go

@@ -0,0 +1,11 @@
+package service
+
+import (
+	"context"
+	"net/http"
+)
+
+// Login 仅执行登录流程，返回已认证的 HTTP 客户端（调用方可继续用它抓取数据）。
+func (s *Service) Login(ctx context.Context, username, password, twfid string) (*http.Client, error) {
+	return s.newAuthenticatedClient(ctx, username, password, twfid)
+}