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/encoding.go

@@ -0,0 +1,75 @@
+package client
+
+import (
+	"bytes"
+	"io"
+	"net/http"
+	"strings"
+
+	"golang.org/x/text/encoding/simplifiedchinese"
+	"golang.org/x/text/transform"
+)
+
+// DecodeResponseBody 读取 HTTP 响应体并按 Content-Type 进行字符集解码（处理 GBK）。
+func DecodeResponseBody(resp *http.Response) (string, error) {
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return "", err
+	}
+	return decodeBody(body, resp.Header.Get("Content-Type")), nil
+}
+
+func decodeBody(body []byte, contentType string) string {
+	lowerCT := strings.ToLower(contentType)
+	if strings.Contains(lowerCT, "charset=gb") || strings.Contains(lowerCT, "charset=gb2312") || strings.Contains(lowerCT, "charset=gbk") {
+		decoded, err := decodeGBK(body)
+		if err == nil {
+			return decoded
+		}
+	}
+
+	if isValidUTF8(body) {
+		return string(body)
+	}
+
+	decoded, err := decodeGBK(body)
+	if err == nil {
+		return decoded
+	}
+	return string(body)
+}
+
+func decodeGBK(data []byte) (string, error) {
+	reader := transform.NewReader(bytes.NewReader(data), simplifiedchinese.GBK.NewDecoder())
+	decoded, err := io.ReadAll(reader)
+	if err != nil {
+		return "", err
+	}
+	return string(decoded), nil
+}
+
+func isValidUTF8(data []byte) bool {
+	for i := 0; i < len(data); i++ {
+		if data[i]&0x80 != 0 {
+			if data[i]&0xE0 == 0xC0 {
+				if i+1 >= len(data) || data[i+1]&0xC0 != 0x80 {
+					return false
+				}
+				i++
+			} else if data[i]&0xF0 == 0xE0 {
+				if i+2 >= len(data) || data[i+1]&0xC0 != 0x80 || data[i+2]&0xC0 != 0x80 {
+					return false
+				}
+				i += 2
+			} else if data[i]&0xF8 == 0xF0 {
+				if i+3 >= len(data) || data[i+1]&0xC0 != 0x80 || data[i+2]&0xC0 != 0x80 || data[i+3]&0xC0 != 0x80 {
+					return false
+				}
+				i += 3
+			} else {
+				return false
+			}
+		}
+	}
+	return true
+}