feat: initial commit — backend API + student cabinet frontend

- Go backend: auth (JWT), points earn/spend, QR token generation,
  partners, admin grant/stats endpoints with chi router
- Next.js 14 frontend: login, student dashboard, transaction history,
  QR display, partners list
- PostgreSQL migrations (4 tables), Redis cache, Docker Compose
- CORS middleware, role-based route protection, Zustand auth store

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/pkg/response/json.go

@@ -0,0 +1,32 @@
+// Package response provides helpers for writing consistent JSON API responses.
+// Every handler must use these helpers — never call json.Encode directly.
+package response
+
+import (
+	"encoding/json"
+	"net/http"
+)
+
+type successBody struct {
+	Data interface{} `json:"data"`
+}
+
+type errorBody struct {
+	Error string `json:"error"`
+}
+
+// JSON writes a successful JSON response with the given HTTP status code and data payload.
+// The payload is wrapped in {"data": ...} to match the API envelope convention.
+func JSON(w http.ResponseWriter, status int, data interface{}) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	json.NewEncoder(w).Encode(successBody{Data: data}) //nolint:errcheck
+}
+
+// Error writes a JSON error response with the given HTTP status code and human-readable message.
+// The message is wrapped in {"error": ...}.
+func Error(w http.ResponseWriter, status int, message string) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	json.NewEncoder(w).Encode(errorBody{Error: message}) //nolint:errcheck
+}