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/exam.go

@@ -0,0 +1,20 @@
+package models
+
+// ExamType 考试类型
+type ExamType string
+
+const (
+	ExamTypeFinal   ExamType = "final"
+	ExamTypeMidterm ExamType = "midterm"
+	ExamTypeMakeup  ExamType = "makeup"
+)
+
+// ExamEntry 考试条目
+type ExamEntry struct {
+	Course  string
+	Date    string
+	Time    string
+	Week    string
+	Weekday string
+	Type    ExamType
+}