Move internal packages to an internal folder

For reference: https://go.dev/doc/go1.4#internalpackages

internal/model/subscription.go

@@ -0,0 +1,15 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package model // import "miniflux.app/v2/internal/model"
+
+// SubscriptionDiscoveryRequest represents a request to discover subscriptions.
+type SubscriptionDiscoveryRequest struct {
+	URL                         string `json:"url"`
+	UserAgent                   string `json:"user_agent"`
+	Cookie                      string `json:"cookie"`
+	Username                    string `json:"username"`
+	Password                    string `json:"password"`
+	FetchViaProxy               bool   `json:"fetch_via_proxy"`
+	AllowSelfSignedCertificates bool   `json:"allow_self_signed_certificates"`
+}