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/empty_classroom.go

@@ -0,0 +1,10 @@
+package models
+
+// EmptyClassroomEntry 空教室条目
+type EmptyClassroomEntry struct {
+	Classroom string `json:"classroom"`
+	Weekday   string `json:"weekday"`
+	Periods   string `json:"periods"`
+	Term      string `json:"term"`
+	Weeks     string `json:"weeks"`
+}