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.

models/grade.go

@@ -0,0 +1,32 @@
+package models
+
+// ScoreType 成绩类型
+type ScoreType string
+
+const (
+	ScoreTypeFinal   ScoreType = "final"
+	ScoreTypeMidterm ScoreType = "midterm"
+)
+
+// GradeEntry 成绩条目
+type GradeEntry struct {
+	Term       string
+	Code       string
+	Name       string
+	Nature     string
+	Category   string
+	Credit     string
+	Score      string
+	GradePoint string
+	IsExam     string
+}
+
+// GPASummary 学分绩概要
+type GPASummary struct {
+	AverageGPA       string
+	Rank             string
+	ExamTotalGPA     string
+	ExamTotalCredits string
+	FailCredits      string
+	Semesters        string
+}