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.

config/config.go

@@ -3,17 +3,14 @@ package config
 import (
 	"log/slog"
 	"os"
-	"strconv"
 	"sync"
 	"time"
 )
 
 type Config struct {
-	BaseURL    string
-	Username   string
-	Password   string
-	Semester   string
-	TWFID      string
+	BaseURL  string
+	Semester string
+	TWFID    string
 	Recognizer RecognizerConfig
 }
 
@@ -33,13 +30,11 @@ func Load() *Config {
 	cfgOnce.Do(func() {
 		cfg = &Config{
 			BaseURL:  getEnv("JWTS_BASE_URL", "http://jwts.hitwh.edu.cn"),
-			Username: getEnv("JWTS_USERNAME", "2024212193"),
-			Password: getEnv("JWTS_PASSWORD", "wyb12580963"),
-			Semester: getEnv("JWTS_SEMESTER", "2025-20262"),
+			Semester: getEnv("JWTS_SEMESTER", ""),
 			TWFID:    getEnv("JWTS_TWFID", ""),
 			Recognizer: RecognizerConfig{
 				APIURL:  getEnv("RECOGNIZER_API_URL", "https://api.littlelan.cn/v1/chat/completions"),
-				APIKey:  getEnv("RECOGNIZER_API_KEY", "sk-6064466699a996b6f83e1ae8247c1cb4eeb4b72ce23cb001204335a2567d63c1"),
+				APIKey:  getEnv("RECOGNIZER_API_KEY", ""),
 				Model:   getEnv("RECOGNIZER_MODEL", "qwen3.5-plus"),
 				Timeout: getEnvDuration("RECOGNIZER_TIMEOUT", 120*time.Second),
 			},
@@ -67,17 +62,6 @@ func getEnv(key, defaultValue string) string {
 	return val
 }
 
-func getEnvInt(key string, defaultValue int) int {
-	val := os.Getenv(key)
-	if val == "" {
-		return defaultValue
-	}
-	if i, err := strconv.Atoi(val); err == nil {
-		return i
-	}
-	return defaultValue
-}
-
 func getEnvDuration(key string, defaultValue time.Duration) time.Duration {
 	val := os.Getenv(key)
 	if val == "" {
@@ -91,8 +75,6 @@ func getEnvDuration(key string, defaultValue time.Duration) time.Duration {
 
 var (
 	BaseURL          = Get().BaseURL
-	Username         = Get().Username
-	Password         = Get().Password
 	Semester         = Get().Semester
 	TWFID            = Get().TWFID
 	RecognizerAPIURL = Get().Recognizer.APIURL