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.

client/fetch.go

@@ -0,0 +1,47 @@
+package client
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"net/http"
+
+	"schedule_converter/config"
+)
+
+// FetchHTML 发送 HTTP 请求并返回解码后的 HTML 字符串（处理 GBK 字符集）。
+// method 为 "GET" 或 "POST"；path 拼接到 config.BaseURL 之后；body 为请求体（GET 传 nil）。
+// referer 为可选的 Referer 路径（拼接到 BaseURL 之后），不需要时传 ""。
+func FetchHTML(ctx context.Context, httpClient *http.Client, method, path string, body io.Reader, contentType, referer string) (string, error) {
+	resp, err := doRequest(ctx, httpClient, method, path, body, contentType, referer)
+	if err != nil {
+		return "", err
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return "", fmt.Errorf("请求 %s 失败, 状态码: %d", path, resp.StatusCode)
+	}
+
+	return DecodeResponseBody(resp)
+}
+
+// doRequest 构造并执行请求，设置 header（可选 Referer），透传 ctx。
+func doRequest(ctx context.Context, httpClient *http.Client, method, path string, body io.Reader, contentType, referer string) (*http.Response, error) {
+	url := config.Get().BaseURL + path
+	req, err := http.NewRequestWithContext(ctx, method, url, body)
+	if err != nil {
+		return nil, err
+	}
+
+	if contentType != "" {
+		req.Header = GetHeadersWithContentType(contentType)
+	} else {
+		req.Header = GetHeaders()
+	}
+	if referer != "" {
+		req.Header.Set("Referer", config.Get().BaseURL+referer)
+	}
+
+	return httpClient.Do(req)
+}