feat: add backend jwt middleware

backend/internal/auth/principal.go

@@ -0,0 +1,27 @@
+package auth
+
+import (
+	"context"
+	"net/http"
+)
+
+type Principal struct {
+	Subject string
+	Email   string
+	Role    string
+}
+
+type principalContextKey struct{}
+
+func PrincipalFromContext(ctx context.Context) (Principal, bool) {
+	principal, ok := ctx.Value(principalContextKey{}).(Principal)
+	return principal, ok
+}
+
+func withPrincipal(ctx context.Context, principal Principal) context.Context {
+	return context.WithValue(ctx, principalContextKey{}, principal)
+}
+
+func PrincipalFromRequest(r *http.Request) (Principal, bool) {
+	return PrincipalFromContext(r.Context())
+}