refactor(grpc): restructure task handling and migrate to slog

Refactor the gRPC client and task handling logic to improve modularity,
error handling, and observability.

- Split `grpc/handler.go` into specialized handler files (login,
  classroom, exams, grades, schedule) to reduce complexity.
- Replace standard `log` with `log/slog` throughout the project for
  structured logging.
- Refactor `grpc/client.go` to use a thread-safe connection management
  pattern with `sync.RWMutex`.
- Remove the legacy `service` package and HTTP server implementation,
  transitioning the application to a pure gRPC runner mode.
- Clean up `config` and `pkg/errors` to remove unused utilities and
  simplify the API.
- Improve error reporting in parsers and HTTP clients by checking
  response status codes.

go.mod

@@ -4,40 +4,14 @@ go 1.25.7
 
 require (
 	github.com/PuerkitoBio/goquery v1.11.0
-	github.com/gin-gonic/gin v1.12.0
+	golang.org/x/text v0.34.0
 	google.golang.org/grpc v1.70.0
 	google.golang.org/protobuf v1.36.10
 )
 
 require (
 	github.com/andybalholm/cascadia v1.3.3 // indirect
-	github.com/bytedance/gopkg v0.1.3 // indirect
-	github.com/bytedance/sonic v1.15.0 // indirect
-	github.com/bytedance/sonic/loader v0.5.0 // indirect
-	github.com/cloudwego/base64x v0.1.6 // indirect
-	github.com/gabriel-vasile/mimetype v1.4.12 // indirect
-	github.com/gin-contrib/sse v1.1.0 // indirect
-	github.com/go-playground/locales v0.14.1 // indirect
-	github.com/go-playground/universal-translator v0.18.1 // indirect
-	github.com/go-playground/validator/v10 v10.30.1 // indirect
-	github.com/goccy/go-json v0.10.5 // indirect
-	github.com/goccy/go-yaml v1.19.2 // indirect
-	github.com/json-iterator/go v1.1.12 // indirect
-	github.com/klauspost/cpuid/v2 v2.3.0 // indirect
-	github.com/leodido/go-urn v1.4.0 // indirect
-	github.com/mattn/go-isatty v0.0.20 // indirect
-	github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd // indirect
-	github.com/modern-go/reflect2 v1.0.2 // indirect
-	github.com/pelletier/go-toml/v2 v2.2.4 // indirect
-	github.com/quic-go/qpack v0.6.0 // indirect
-	github.com/quic-go/quic-go v0.59.0 // indirect
-	github.com/twitchyliquid64/golang-asm v0.15.1 // indirect
-	github.com/ugorji/go/codec v1.3.1 // indirect
-	go.mongodb.org/mongo-driver/v2 v2.5.0 // indirect
-	golang.org/x/arch v0.22.0 // indirect
-	golang.org/x/crypto v0.48.0 // indirect
 	golang.org/x/net v0.51.0 // indirect
 	golang.org/x/sys v0.41.0 // indirect
-	golang.org/x/text v0.34.0 // indirect
 	google.golang.org/genproto/googleapis/rpc v0.0.0-20241202173237-19429a94021a // indirect
 )